// https://cirosantilli.com/linux-kernel-module-cheat#cpp-decltype

#include <cassert>

#include <vector>

#include <utility> // declval

int f() {

return 1;

}

class C {

public:

int f() { return 2; }

};

int i;

decltype(i) g() {

return 1;

}

int main() {

#if __cplusplus >= 201103L

// Implies reference while auto does not.

{

int i = 0;

int& ir = i;

decltype(ir) ir2 = ir;

ir2 = 1;

assert(i == 1);

}

// Can be used basically anywhere.

{

int i = 0;

std::vector<decltype(i)> v;

v.push_back(0);

}

// Return value.

{

decltype(f()) i;

assert(typeid(i) == typeid(int));

C c;

decltype(c.f()) j;

assert(typeid(j) == typeid(int));

// Return value without instance. Use declval.

// http://stackoverflow.com/questions/9760358/decltype-requires-instantiated-object

decltype(std::declval<C>().f()) k;

assert(typeid(k) == typeid(int));

}

// decltype must take expressions as input, not a type.

// For types with the default constructor like `int`, we can just call the default constructor as in int():

// https://stackoverflow.com/questions/39279074/what-does-the-void-in-decltypevoid-mean-exactly

// or we can just use declval.

{

decltype(int()) i = 0;

assert(typeid(i) == typeid(int));

decltype(std::declval<int>()) j = 0;

assert(typeid(j) == typeid(int));

}

// Can be used to declare the return value of functions.

assert(g() == 1);

#endif

}