如何在另一个类中调用自定义构造函数

Question

If I have 2 classes, one with a custom constructor, and the other with an instance of the first class. How do I create that instance with the custom constructor.

For example:

ah

class A
{
public:
    A(std::string input);
};

bh

Class B
{
public:
    A a("Greetings");
};

This wouldn't work properly, it gives the error "expected a type specifier" on the string itself, and whenever I use a member of class A in class B, it says "expression must have a class type"

I'm assuming this means I'd need to make it

A a(std::string words);

But I'm not sure where or how I would define what the string should be.

Answer 1

Use the constructor's initialization list :

class A
{
public:
    A (std::string input);
};

class B
{
    A a;
public:
    B (std::string s) : a (s) {}; //This calls the constructor of A on 'a'
};

Also, in C++11 you can use the uniform initializer syntax :

class B
{
    A a {"Greetings"}.
    ...
};

But with this, you can only call the constructor with a compile-time constant.