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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="common-compile-linker-errors-gcc"><a href="#common-compile-linker-errors-gcc" class="header-anchor">#</a> Common compile/linker errors (GCC)</h1> <h2 id="undefined-reference-to"><a href="#undefined-reference-to" class="header-anchor">#</a> undefined reference to `***'</h2> <p>This linker error happens, if the linker can't find a used symbol.
Most of the time, this happens if a used library is not linked against.</p> <p><strong>qmake:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>LIBS <span class="token operator">+=</span> nameOfLib
</code></pre></div><p><strong>cmake:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token function">TARGET_LINK_LIBRARIES</span><span class="token punctuation">(</span>target nameOfLib<span class="token punctuation">)</span>
</code></pre></div><p><strong>g++ call:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>g<span class="token operator">++</span> <span class="token operator">-</span>o main main<span class="token punctuation">.</span>cpp <span class="token operator">-</span>Llibrary<span class="token operator">/</span>dir <span class="token operator">-</span>lnameOfLib
</code></pre></div><p>One might also forget to compile and link all used <code>.cpp</code> files (functionsModule.cpp defines the needed function):</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>g<span class="token operator">++</span> <span class="token operator">-</span>o binName main<span class="token punctuation">.</span>o functionsModule<span class="token punctuation">.</span>o
</code></pre></div><h2 id="error-was-not-declared-in-this-scope"><a href="#error-was-not-declared-in-this-scope" class="header-anchor">#</a> error: '***' was not declared in this scope</h2> <p>This error happens if a unknown object is used.</p> <h3 id="variables"><a href="#variables" class="header-anchor">#</a> Variables</h3> <p>Not compiling:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><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 keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span>argv<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 keyword">int</span> i <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> i <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span> <span class="token comment">// i is not in the scope of the main function</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Fix:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><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 keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span>argv<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 keyword">int</span> i <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> i <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h3 id="functions"><a href="#functions" class="header-anchor">#</a> Functions</h3> <blockquote></blockquote> <p>Most of the time this error occurs if the needed header is not
included (e.g. using `std::cout` without `#include <iostream>`)</iostream></p> <p>Not compiling:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><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 keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span>argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"No!"</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Fix:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><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 keyword">void</span> <span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// forward declare the function</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span>argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"No!"</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Or:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><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 keyword">void</span> <span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// define the function before using it</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"No!"</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<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 keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span>argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token function">doCompile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>Note:</strong> The compiler interprets the code from top to bottom (simplification). Everything must be at least <a href="http://www.cprogramming.com/declare_vs_define.html" target="_blank" rel="noopener noreferrer">declared (or defined)<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> before usage.</p> <h2 id="fatal-error-no-such-file-or-directory"><a href="#fatal-error-no-such-file-or-directory" class="header-anchor">#</a> fatal error: ***: No such file or directory</h2> <p>The compiler can't find a file (a source file uses <code>#include "someFile.hpp"</code>).</p> <p><strong>qmake:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>INCLUDEPATH <span class="token operator">+=</span> dir<span class="token operator">/</span>Of<span class="token operator">/</span>File
</code></pre></div><p><strong>cmake:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token function">include_directories</span><span class="token punctuation">(</span>dir<span class="token operator">/</span>Of<span class="token operator">/</span>File<span class="token punctuation">)</span>
</code></pre></div><p><strong>g++ call:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>g<span class="token operator">++</span> <span class="token operator">-</span>o main main<span class="token punctuation">.</span>cpp <span class="token operator">-</span>Idir<span class="token operator">/</span>Of<span class="token operator">/</span>File
</code></pre></div></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/common-compile-linker-errors-gcc.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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