Question

Ok, I’m usually all right at being able to read, understand and fix compiler errors. But with this one, I think I need help.

I want to have a std::basic_string<CustomClass> where CustomClass is a class. I don’t want to write custom char_traits and allocator classes for it, unless absolutely necessary (i.e. I want to use std::char_traits<CustomClass> and std::allocator<CustomClass> if possible).

It compiles fine if I have no constructors in CustomClass. As soon as I add one, there are errors:

Call to implicitly-deleted default constructor of 'std::__1::basic_string<CustomClass, std::__1::char_traits<CustomClass>, std::__1::allocator<CustomClass> >::__rep'

    #include <iostream>
    #include <string>
    //#include <vector>

    class CustomClass;

    typedef std::basic_string<CustomClass> InstanceString;
    typedef std::basic_string<int> IntString;

    class CustomClass
    {
    public:
        CustomClass()
            : m_X()
        {
        }

        CustomClass(const int x)
            : m_X(x)
        {
        }

    private:
        int     m_X;
    };

    int main(int argc, const char * argv[])
    {
        // This compiles fine
        IntString s1({1, 2, 5});

        // This would compile fine if there were no explicit constructors in Instance
        //InstanceString s2e = InstanceString({Instance(), Instance(), Instance()});

        // This generates errors
        InstanceString s2 = InstanceString({CustomClass(1), CustomClass(3), CustomClass(5)});

        std::cout << "Hello, World!\n";
        return 0;
    }

I understand this probably has to do with implicit/explicit constructors, copy/move semantics and stuff like that.

My question is:

• how do I get it to compile (i.e. what constructors/something should I add to the class)
• and how do I systematically figure out how to fix these types of compilation errors?

Answer 1

As you said, the error message says

Call to implicitly-deleted default constructor of 'std::__1::basic_string, std::__1::allocator >::__rep'

I’m pretty sure that __rep here is your CustomClass. It’s saying it’s trying to call the default constructor, but that’s been implicitly deleted (by you providing your own constructor). I’m guessing that basic_string uses std::is_default_constructible<>. As such, you need to provide a default constructor using

CustomClass() = default;

as Mooing Duck suggested in the comments.

It seems likely that it actually uses std::is_trivially_default_constructible<>, which imposes the restriction that your class must be trivially constructible as well.