C++ Variable Declaration Keywords decltype
Example
C++11
Yields the type of its operand, which is not evaluated.
-
If the operand
eis a name without any additional parentheses, thendecltype(e)is the declared type ofe.int x = 42; std::vector<decltype(x)> v(100, x); // v is a vector<int> -
If the operand
eis a class member access without any additional parentheses, thendecltype(e)is the declared type of the member accessed.struct S { int x = 42; }; const S s; decltype(s.x) y; // y has type int, even though s.x is const -
In all other cases,
decltype(e)yields both the type and the value category of the expressione, as follows:- If
eis an lvalue of typeT, thendecltype(e)isT&. - If
eis an xvalue of typeT, thendecltype(e)isT&&. - If
eis a prvalue of typeT, thendecltype(e)isT.
This includes the case with extraneous parentheses.
int f() { return 42; } int& g() { static int x = 42; return x; } int x = 42; decltype(f()) a = f(); // a has type int decltype(g()) b = g(); // b has type int& decltype((x)) c = x; // c has type int&, since x is an lvalue - If
C++14
The special form decltype(auto) deduces the type of a variable from its initializer or the return type of a function from the return statements in its definition, using the type deduction rules of decltype rather than those of auto.
const int x = 123;
auto y = x; // y has type int
decltype(auto) z = x; // z has type const int, the declared type of x
