decltype in c++

decltype is a keyword in C++ that is used to determine the type of an expression at compile-time. It can be used to declare variables with the same type as an existing expression or to deduce the return type of a function.

Here are the steps to use decltype in C++:

Declare a variable with the same type as an existing expression: To declare a variable with the same type as an existing expression, you can use the decltype keyword followed by parentheses containing the expression. For example:

int x = 5;
decltype(x) y; // y is declared with the same type as x (int)

In this example, y is declared with the same type as x, which is int.

Deduce the return type of a function: decltype can also be used to deduce the return type of a function. This is particularly useful when the return type depends on the types of the function's arguments. For example:

template <typename T, typename U>
auto add(T a, U b) -> decltype(a + b)
{
    return a + b;
}

In this example, the decltype keyword is used to deduce the return type of the add function based on the types of its arguments a and b. The return type will be the same as the type of the expression a + b.

Accessing the type of a variable without initializing it: decltype can also be used to obtain the type of a variable without initializing it. This can be useful in certain situations, such as when you need to declare another variable with the same type. For example:

int x = 5;
decltype(x) y; // y is declared with the same type as x (int), but not initialized

In this example, decltype(x) is used to obtain the type of x without initializing y.

Overall, decltype is a useful keyword in C++ that allows you to determine the type of an expression at compile-time. It can be used to declare variables with the same type as an existing expression or to deduce the return type of a function.