C ++模板可以推断返回类型吗？

Question

I'm playing around with templates and I'm wondering if there's any way to make code like this work.

template <typename T>
T foo (int a)
{
  return a * 2;
}

int something = foo (123);

The problem here is that the type cannot be inferred by the compiler.

I know that this would work if I this used in the above case.

int a = foo <int> (123);

or even

template <typename T>
T foo (T a)
{
  return a * 2;
}
int a = foo (123);

EDIT: For clarification, I'd like to know if it's possible to make the code return a double when used like this double x = foo (123); and an int when used like this int x = foo (123); .

Answer 1

One way to infer a return type (although it's unclear what you're going to use that for) is to use a templated conversion, eg

class foo
{
private:
    int a_;
public:
    template< class Return_type >
    operator Return_type () const
    { return a_*2; }

    foo( int const a ): a_( a ) {}
};

void bar()
{ int a = foo( 123 ); }

Disclaimer: code untouched by compiler's hands.

Answer 2

Using type_traits , you can do all this and more:

#include <type_traits>    

template<typename T = std::enable_if<std::is_arithmetic<T>::value>::type>
T foo(T a)
{
    return a * 2;
}

int main()
{
    auto bar = foo(1); //bar is an int
    auto car = foo(1.0) //car is a double
    auto star = foo(1.0f); //star is a float
}