CODEWITHSUNDEEP
c++templatesc++11
Cfon
Cfon

Reputation: 245

C++11: function template: passing parameters by reference

There is a kind of template function to pass a parameter by reference + callback function, but there is a problem if the parameters are passed by reference to callback function then compiler is throwing an error:

no matching function for call to func(int&, void (&)(int&)).

What's wrong?

template<typename T> 
using func_t = void(T);

template<typename T>
void func(T& arg, func_t<T> callback) {
    callback(arg);
} 

void func1(int arg) {  }
void func2(int& arg) { } //<-- (1)

int main() {
    int x = 0;
    func(x, func1);
    func(x, func2); //<-- (2) compilation error 
}

Upvotes: 4

Views: 159

Answers (1)

Brian Bi
Brian Bi

Reputation: 119174

Deduction of T in the second call fails because T occurs in two deduced contexts which deduce different values of T.

In the first parameter, T& arg, T is deduced to int since the argument x has type int.

In the second parameter, func_t<T>, T is deduced to int& since the argument func2 has type void(int&).

Since the two deduced types, int and int&, differ for the same parameter, deduction fails.

However, you can do the right thing by explicitly specifying that T should be int&:

func<int&>(x, func2); // ok

Upvotes: 6

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