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c++constructorinitialization

C++17 why is there no default constructor provided when passing in arguments

I have this class:

class MyClass {
public:
    int someMember;
    std::vector<int> someVectorMember;
};

and I was wondering why this wouldn't work:

MyClass(1, std::vector<int>{1,2});

Error:

main.cpp:21:37: error: no matching function for call to ‘MyClass::MyClass(int, std::vector)’
Solution

  • The problem is that you're passing two arguments of which the first is of type int and the second is of type std::vector<int> while there is no such constructor that takes two parameters of the mentioned types. Note also that in C++17, MyClass aggregate initialization cannot work using parenthesis.

    In C++20 though aggregate initialization can use parenthesis so this will work in c++20 as per p0960: Allow initializing aggregates from a parenthesized list of values .

    why is there no default constructor provided

    There is a default ctor MyClass::MyClass() implicitly generated by the compiler, it's just that it cannot be used for MyClass(1, std::vector<int>{1,2})