In-class constructor initialisation of a vector in C-CodePudding

I have class MyClass as below. I would like to initilaise the attributes of MyClass through a constructor. The arguments b_ and c_ should have the defaults {"abc", "_"} and 0 respectively.

class MyClass {

    size_t a_;
    std::vector<string> b_;
    size_t c_;

public:

    MyClass(
            size_t a,
            optional<std::vector<string>> b = {"abc", "_"},
            size_t c = 0)
        :   a_(a),
            b_(b),
            c_(c)
        {}
}

This gives me the error: error: could not convert ‘{"abc", "_"}’ from ‘<brace-enclosed initializer list>’ to ‘isf::optional<std::vector<std::__cxx11::basic_string<char> > >’.

I then tried:

class MyClass {

    size_t a_;
    std::vector<string> b_;
    size_t c_;

public:
    const std::vector<string> VEC = {"abc", "_"};

    MyClass(
            size_t a,
            optional<std::vector<string>> b = VEC,
            size_t c = 0)
        :   a_(a),
            b_(b),
            c_(c)
        {}
}

This gave me the error: error: invalid use of non-static data member ‘MyClass::VEC’.

Making VEC static also did not solve the issue: error: in-class initialization of static data member ‘const std::vector<std::__cxx11::basic_string<char> > MyClass::b’ of non-literal type

What would be the correct way of declaring a default value for the vector of strings in the constructor?

CodePudding user response：

The right way is to use a std::vector<std::string>>:

MyClass(
            size_t a,
            std::vector<std::string> b = {"abc", "_"},
            size_t c = 0)
        :   a_(a),
            b_(b),
            c_(c)
        {}

There is no use of optional here, because either the caller does pass a vector or the caller does not pass a vector but then the default is used. As you want to initialize the member always, passing nullopt is not an option either. Hence, just drop the optional.

CodePudding user response：

It can be solved in the following way:

class MyClass {

    size_t a_;
    std::vector<string> b_;
    size_t c_;

public:
    const std::vector<string> DEFAULT_VEC = {"abc", "_"};

    MyClass(
            size_t a,
            optional<std::vector<string>> b,
            size_t c = 0)
        :   a_(a),
            b_(b.value_or(DEFAULT_VEC)),
            c_(c)
        {}
}