Can C deduce argument type from default value?-CodePudding

I tried to write this function with a default template argument:

template<typename A, typename B>
void func(int i1, int i2, A a, B b = 123){
    ...
}

In my mind I can call it like this: func(1, 2, 3) and compiler should deduce type B as int from default value, but I get no instance of overloaded function.
Is it incorrect C construction and compiler can't deduce type in this case?

CodePudding user response：

Yes. Template parameter can't be deduced from function default argument.

Type template parameter cannot be deduced from the type of a function default argument:

template<typename T> void f(T = 5, T = 7); 

void g()
{
    f(1);     // OK: calls f<int>(1, 7)
    f();      // error: cannot deduce T
    f<int>(); // OK: calls f<int>(5, 7)
}

You can specify default argument for the template parameter too.

template<typename A, typename B = int>
void func(int i1, int i2, A a, B b = 123){
    ...
}

CodePudding user response：

As often when default arguments don't work you can use overloads:

template<typename A, typename B>
void func(int i1, int i2, A a, B b){
    ...
}
template<typename A>
void func(int i1, int i2, A a){
    func(i1,i2,a,123);
}