Trying to understand the different ways of initializing std::string

Question

std::string str;
std::string(str);
std::string str = {};

Are they the same? If no then what's the difference? And when should I use each of them?

New to C++ and trying to understand the different ways of initializing std::string.

Answer 1

These two declarations

std::string str;
std::string(str);

are equivalent. A declarator may be enclosed in parentheses.

Here is an example of an array declaration.

int ( ( ( a )[N1] )[N2] );

though the parentheses are redundant.

As for this declaration

std::string str = {};

then there is used the copy-initialization that requires that the default constructor would be acceptable. If it is declared as explicit (for the class std::string it is not explicit) then the compiler will issue an error.

Here is a demonstration program that produces a compiler error.

int main()
{
    struct A
    {
        explicit A() = default;
    };

    A a = {};
}

If in the demonstration program to change the declaration of the object a from

A a = {};

to

A a{};

then the program will compile because in that case there will not be the copy-initialization. There will be used the direct-initialization.

As for the original declarations then they all use the default constructor of the class std::string that is not declared as explicit. So all three declarations have the same effect.