Question

Lets say I have a function which takes a function pointer as a parameter, and that parameter has a default argument.

template <typename T>
T* default_construct()
{
    return new T();
}

template <typename T>
void register(T* (*construct)() = default_construct<T>)
{
    // Save that function pointer for later
}

Lets say I want to use register on my class Foo, but Foo doesn’t have a default constructor, so my default_construct won’t work on it. The obvious solution is to do something like this:

Foo* construct_Foo()
{
    return new Foo("String argument", 123);
}

SomeFunc()
{
    // ...
    register<Foo>(construct_Foo);
    // ...
}

But that doesn’t work. Even though register<Foo> may only be called in one place, and it’s passed a function to use, default_construct<Foo> still gets instantiated by the compiler, and I get compiler errors. It seems like since it never gets used, it ought to be skipped over, but I guess that’s not the case.

Is there any way to prevent default_construct from being instantiated when it’s being used as a default argument? The only solution I can think of is to put it in the template, but it seems like there ought to be a better solution.

Answer 1

Here’s one solution that solves the problem because it doesn’t use default arguments:

template <typename T>
T* default_construct()
{
    return new T();
}

template <typename T>
void register(T* (*construct)())
{
    // Save that function pointer for later
}

template<typename T>
void register()
{
    register<T>(default_construct<T>);
}

Note that register is a C++ keyword though 🙂