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vs C++17</a></li><li><a href="/cpp/compiling-and-building.html" class="sidebar-link">Compiling and Building</a></li><li><a href="/cpp/common-compile-linker-errors-gcc.html" class="sidebar-link">Common compile/linker errors (GCC)</a></li><li><a href="/cpp/more-undefined-behaviors-in-c.html" class="sidebar-link">More undefined behaviors in C++</a></li><li><a href="/cpp/unit-testing-in-c.html" class="sidebar-link">Unit Testing in C++</a></li><li><a href="/cpp/c-debugging-and-debug-prevention-tools-techniques.html" class="sidebar-link">C++ Debugging and Debug-prevention Tools & Techniques</a></li><li><a href="/cpp/optimization-in-c.html" class="sidebar-link">Optimization in C++</a></li><li><a href="/cpp/optimization.html" class="sidebar-link">Optimization</a></li><li><a href="/cpp/profiling.html" class="sidebar-link">Profiling</a></li><li><a href="/cpp/refactoring-techniques.html" class="sidebar-link">Refactoring Techniques</a></li><li><a href="/cpp/internationalization-in-c.html" class="sidebar-link">Internationalization in C++</a></li><li><a href="/cpp/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="copying-vs-assignment"><a href="#copying-vs-assignment" class="header-anchor">#</a> Copying vs Assignment</h1> <h2 id="assignment-operator"><a href="#assignment-operator" class="header-anchor">#</a> Assignment Operator</h2> <p>The Assignment Operator is when you replace the data with an already existing(previously initialized) object with some other object's data. Lets take this as an example:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// Assignment Operator</span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><string></span></span>
<span class="token keyword">using</span> std<span class="token double-colon punctuation">::</span>cout<span class="token punctuation">;</span>
<span class="token keyword">using</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">Foo</span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">Foo</span><span class="token punctuation">(</span><span class="token keyword">int</span> data<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token operator">-></span>data <span class="token operator">=</span> data<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token operator">~</span><span class="token function">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Foo<span class="token operator">&</span> <span class="token keyword">operator</span><span class="token operator">=</span><span class="token punctuation">(</span><span class="token keyword">const</span> Foo<span class="token operator">&</span> rhs<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
data <span class="token operator">=</span> rhs<span class="token punctuation">.</span>data<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token operator">*</span><span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> data<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
Foo <span class="token function">foo</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//Foo(int data) called</span>
Foo <span class="token function">foo2</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
foo <span class="token operator">=</span> foo2<span class="token punctuation">;</span> <span class="token comment">// Assignment Operator Called</span>
cout <span class="token operator"><<</span> foo<span class="token punctuation">.</span>data <span class="token operator"><<</span> endl<span class="token punctuation">;</span> <span class="token comment">//Prints 42</span>
<span class="token punctuation">}</span>
</code></pre></div><p>You can see here I call the assignment operator when I already initialized the <code>foo</code> object. Then later I assign <code>foo2</code> to <code>foo</code> . All the changes to appear when you call that equal sign operator is defined in your <code>operator=</code> function. You can see a runnable output here: <a href="http://cpp.sh/3qtbm" target="_blank" rel="noopener noreferrer">http://cpp.sh/3qtbm<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <h2 id="copy-constructor"><a href="#copy-constructor" class="header-anchor">#</a> Copy Constructor</h2> <p>Copy constructor on the other hand , is the complete opposite of the Assignment Constructor. This time, it is used to initialize an already nonexistent(or non-previously initialized) object. This means it copies all the data from the object you are assigning it to , without actually initializing the object that is being copied onto. Now Let's take a look at the same code as before but modify the assignment constructor to be a copy constructor :</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// Copy Constructor</span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><string></span></span>
<span class="token keyword">using</span> std<span class="token double-colon punctuation">::</span>cout<span class="token punctuation">;</span>
<span class="token keyword">using</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">Foo</span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">Foo</span><span class="token punctuation">(</span><span class="token keyword">int</span> data<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token operator">-></span>data <span class="token operator">=</span> data<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token operator">~</span><span class="token function">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token function">Foo</span><span class="token punctuation">(</span><span class="token keyword">const</span> Foo<span class="token operator">&</span> rhs<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
data <span class="token operator">=</span> rhs<span class="token punctuation">.</span>data<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> data<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
Foo <span class="token function">foo</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//Foo(int data) called</span>
Foo foo2 <span class="token operator">=</span> foo<span class="token punctuation">;</span> <span class="token comment">// Copy Constructor called</span>
cout <span class="token operator"><<</span> foo2<span class="token punctuation">.</span>data <span class="token operator"><<</span> endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>You can see here <code>Foo foo2 = foo;</code> in the main function I immediately assign the object before actually initializing it, which as said before means it's a copy constructor. And notice that I didn't need to pass the parameter int for the <code>foo2</code> object since I automatically pulled the previous data from the object foo. Here is an example output : <a href="http://cpp.sh/5iu7" target="_blank" rel="noopener noreferrer">http://cpp.sh/5iu7<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <h2 id="copy-constructor-vs-assignment-constructor"><a href="#copy-constructor-vs-assignment-constructor" class="header-anchor">#</a> Copy Constructor Vs Assignment Constructor</h2> <p>Ok we have briefly looked over what the copy constructor and assignment constructor are above and gave examples of each now let's see both of them in the same code. This code will be similar as above two. Let's take this :</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// Copy vs Assignment Constructor</span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><string></span></span>
<span class="token keyword">using</span> std<span class="token double-colon punctuation">::</span>cout<span class="token punctuation">;</span>
<span class="token keyword">using</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">Foo</span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">Foo</span><span class="token punctuation">(</span><span class="token keyword">int</span> data<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token operator">-></span>data <span class="token operator">=</span> data<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token operator">~</span><span class="token function">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token function">Foo</span><span class="token punctuation">(</span><span class="token keyword">const</span> Foo<span class="token operator">&</span> rhs<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
data <span class="token operator">=</span> rhs<span class="token punctuation">.</span>data<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
Foo<span class="token operator">&</span> <span class="token keyword">operator</span><span class="token operator">=</span><span class="token punctuation">(</span><span class="token keyword">const</span> Foo<span class="token operator">&</span> rhs<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
data <span class="token operator">=</span> rhs<span class="token punctuation">.</span>data<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token operator">*</span><span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> data<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
Foo <span class="token function">foo</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//Foo(int data) / Normal Constructor called</span>
Foo foo2 <span class="token operator">=</span> foo<span class="token punctuation">;</span> <span class="token comment">//Copy Constructor Called</span>
cout <span class="token operator"><<</span> foo2<span class="token punctuation">.</span>data <span class="token operator"><<</span> endl<span class="token punctuation">;</span>
Foo <span class="token function">foo3</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
foo3<span class="token operator">=</span>foo<span class="token punctuation">;</span> <span class="token comment">//Assignment Constructor Called</span>
cout <span class="token operator"><<</span> foo3<span class="token punctuation">.</span>data <span class="token operator"><<</span> endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Output:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token number">2</span>
<span class="token number">2</span>
</code></pre></div><p>Here you can see we first call the copy constructor by executing the line <code>Foo foo2 = foo;</code> . Since we didn't initialize it previously. And then next we call the assignment operator on foo3 since it was already initialized <code>foo3=foo</code>;</p> <h4 id="syntax"><a href="#syntax" class="header-anchor">#</a> Syntax</h4> <ul><li><strong>Copy Constructor</strong></li> <li>MyClass( const MyClass& other );</li> <li>MyClass( MyClass& other );</li> <li>MyClass( volatile const MyClass& other );</li> <li>MyClass( volatile MyClass& other );</li> <li><strong>Assignment Constructor</strong></li> <li>MyClass& operator=( const MyClass& rhs );</li> <li>MyClass& operator=( MyClass& rhs );</li> <li>MyClass& operator=( MyClass rhs );</li> <li>const MyClass& operator=( const MyClass& rhs );</li> <li>const MyClass& operator=( MyClass& rhs );</li> <li>const MyClass& operator=( MyClass rhs );</li> <li>MyClass operator=( const MyClass& rhs );</li> <li>MyClass operator=( MyClass& rhs );</li> <li>MyClass operator=( MyClass rhs );</li></ul> <h4 id="parameters"><a href="#parameters" class="header-anchor">#</a> Parameters</h4> <table><thead><tr><th>rhs</th> <th>Right Hand Side of the equality for both copy and assignment constructors. For example the assignment constructor : MyClass operator=( MyClass& rhs );</th></tr></thead> <tbody><tr><td>Placeholder</td> <td>Placeholder</td></tr></tbody></table> <h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p>Other Good Resources for further research :</p> <p><a href="http://stackoverflow.com/questions/11706040/whats-the-difference-between-assignment-operator-and-copy-constructor" target="_blank" rel="noopener noreferrer">What's the difference between assignment operator and copy constructor?<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p><a href="http://stackoverflow.com/questions/18969083/assignment-operator-vs-copy-constructor-c" target="_blank" rel="noopener noreferrer">assignment operator vs. copy constructor C++<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p><a href="http://www.geeksforgeeks.org/copy-constructor-vs-assignment-operator-in-c/" target="_blank" rel="noopener noreferrer">GeeksForGeeks<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p><a href="http://www.cplusplus.com/articles/y8hv0pDG/" target="_blank" rel="noopener noreferrer">C++ Articles<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/copying-vs-assignment.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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