如何使用已知参数初始化 object，该参数是 C++ 中 class 的成员？

Question

I have the following class:

class Counter {
private:
    unsigned int count;
    unsigned int inc_by;

public:
    Counter(unsigned int count, unsigned int inc_by) : count(count), inc_by(inc_by) {}

    void increment() {
        count += inc_by;
    }
};

I think have another class Timer where I want to use the Counter object in. However, I want every instance of Timer to have a Counter object private member that is initialized with known parameters. I also can not use dynamic memory allocation.

I have tried the following:

class Timer {
private:
    Counter counter(0, 1);

public:
    Timer() {}
};

This results in the compiler error Function 'counter' is not implemented .

What am I doing wrong?

Answer 1

Write

Counter counter { 0, 1 };

You may use only the so-called brace-or-equal-initializers.

Or you could initialize the data member in the constructor like

Timer() : counter( 0, 1 ){}

Answer 2

There are only two ways you can initialize non-static data members.

If you want to give an initial value to a member where it's declared, you need {} , like this:

Counter counter {0, 1};

Otherwise, you can define the initial value in the member initializer list of Timer 's constructor, like this:

Timer() : counter(0,1) {}