Declval (example of return value of guidance function)

There are the following large codes ：

int myfunc(int a, int b)
{
    
	return a + b;
}

template<typename T_F,typename...U_Args>
decltype(declval<T_F>() (declval<U_Args>()...)) testImpl(T_F func, U_Args...args)
{
    

	return func(args...);
}

int main()
{
    	
	int nSum = testImpl(myfunc, 13, 15);
	cout << nSum << endl;
	system("pause");
	return 0;
}

Running results ：

We all know that the function type is determined by the return value type and parameter type , It has nothing to do with the function name , So here myfunc The function type of is int(int,int), The previous represents the return value type , In parentheses are two parameter types .

The above code has an interesting code for deriving the return value type of function template . from main You can see in the function ,testImpl Template parameters are not specified , So the template parameters are inferred by the compiler . Here is a simple explanation of inference types .
（1）T_F To be inferred as int(*)(int,int)
（2）U_Args Represents a package type , What's coming in is 5,8, So it will be deduced as int,int.
（3） It is worth noting that decltype(declval<T_F>() (declval<U_Args>()...)), This kind of writing , I was not used to it when I first came into contact , But just figure out its function ： Deduce the return value type of the function according to the function type and parameter type .

Actually , There's another way not to use std::declval It can also achieve the same function - Return type postposition . It's more practical than std::declval It's so refreshing .

template<typename T_F, typename...U_Args>
template<typename T_F, typename...U_Args>
auto testImpl_2(T_F func, U_Args...args)->decltype(func(args...))
{
    

	return func(args...);
}

int main()
{
    	
	int nSum = testImpl_2(myfunc, 13, 15);
	cout << nSum << endl;
	system("pause");
	return 0;
}

result ：28

Maybe some students , Will take out ->decltype(func(args...)), Because this can also get the right answer , But it's not a good idea ,testImpl_2 The original meaning of is the type you want to return and the first type template parameter T_F Type represented （ Both myfunc() Function return type ） Exactly the same as , And that's what “->decltype(func(args…))” The meaning of code .