C++ 是否优化了对象创建？

Question

If I have a class with two constructors like this:

class Test {

    public:

        Test(int x) {
            _num = x;
        }

        Test() {
            _num = 0;
        }

    private:

        int _num;
};

I want to create a stack object based on a condition like this:

    Test test;
    if (someCondition() == 23) {
        test = Test(42);
    }

Will I have the overhead of creating a Test object two times, calling both constructors, in this case? Or will this be optimized out in general? Is this considered good practice?

Toy-examples in compiler explorer are optimized heavily with inlining with no apparent constructor call left. So it's not really clear to me.

Answer 1

Write code to express intent.

You do not want to call the constuctor twice. Don't call the constructor twice:

Test test = (someCondition() == 23) ? Test() : Test(42);

Can the compiler optimize your code to have only one Test constructed?

Yes. Compiler optimizations must follow the so-called "as-if rule" . In a nutshell: Creating two, one, or none, or 100 instances in your code has no observable effect. The compiler can notice that and produce a program where no instance is created.

Will the compiler optimize your code to have only one Test constructed?

You cannot tell unless you try (or you are a compiler writer and your brain is capable of doing the job of a compiler... then you can know without trying). That's why it is important to write code to express intent. The code you write is not instructions for the CPU, but instructions for the compiler to generate instructions for the CPU.