c++ - Strange code segment from c++

Question

Reading code from other posts, I'm seeing something like this.

struct Foo {
  Foo() : mem(0) {}
  int mem;
};

What does mem(0) {} does in this case, especially regarding the curly brackets? I have never seen this before and have no idea where else I would find out about this. I know that mem(0), would intialize mem to 0, but why the {}?

Thanks.

Answer 1

Since Foo() is the class' constructor, it must have a body, even if the member variable mem is initialized outside of it.

That's why, in your example, the constructor has an empty body:

Foo() : mem(0)
{
    // 'mem' is already initialized, but a body is still required.
}

Answer 2

It defines the constructor of the class. The part after the colon is the initialization list, in which the mem member is initialized to zero using a constructor call.

Compare:

int a(0);
int b = 0;

These two do the same, but the former is more in line with how object construction typically looks in C++.

Answer 3

int c++ 你可以在 .h 文件中定义你的方法实现

class MyClass
{
  public:
   MyClass(){
     .....
   }

   void doSomething(){
     .....
   }

   ~MyClass(){
     .....
   }
};

通常它用于模板实现。如果您想避免库链接，并且您更愿意将所有代码提供给用户，您也可以使用这种类声明方法，这样他就可以包含您的文件，而无需将任何 lib 文件链接到他的项目。