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Copy pathdecltype.cpp
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61 lines (48 loc) · 1.83 KB
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#include <iostream>
#include <type_traits>
#include <stdio.h>
#include <typeinfo>
/*
* Inspects the declared type of an entity or the type and value category of an expression.
*
*
*/
struct A { double x; };
const A* a;
decltype(a->x) y; // type of y is double (declared type)
decltype((a->x)) z = y; // type of z is const double& (lvalue expression)
template<typename T, typename U>
auto add(T t, U u) -> decltype(t + u) // return type depends on template parameters
// return type can be deduced since C++14
{
return t + u;
}
const int& getRef(const int* p) { return *p; }
static_assert(std::is_same_v<decltype(getRef), const int&(const int*)>);
auto getRefFwdBad(const int* p) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdBad), int(const int*)>,
"Just returning auto isn't perfect forwarding.");
decltype(auto) getRefFwdGood(const int* p) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdGood), const int&(const int*)>,
"Returning decltype(auto) perfectly forwards the return type.");
// Alternatively:
auto getRefFwdGood1(const int* p) -> decltype(getRef(p)) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdGood1), const int&(const int*)>,
"Returning decltype(return expression) also perfectly forwards the return type.");
int main()
{
int i = 33;
decltype(i) j = i * 2;
std::cout << "i and j are the same type? " << std::boolalpha << std::is_same_v<decltype(i), decltype(j)> << '\n';
std::cout << "i = " << i << ", "
<< "j = " << j << '\n';
auto f = [](int a, int b) -> int
{
return a * b;
};
decltype(f) g = f; // the type of a lambda function is unique and unnamed
i = f(2, 2);
j = g(3, 3);
std::cout << "i = " << i << ", "
<< "j = " << j << '\n';
}