c++ - 如何在类中初始化 const 成员变量？

Question

#include <iostream>

using namespace std;
class T1
{
  const int t = 100;
  public:

  T1()
  {

    cout << "T1 constructor: " << t << endl;
  }
};

当我尝试用 100 初始化 const 成员变量 t 时。但它给了我以下错误:

test.cpp:21: error: ISO C++ forbids initialization of member ‘t’
test.cpp:21: error: making ‘t’ static

如何初始化 const 值？

Answer 1

const 变量指定变量是否可修改。每次引用变量时都将使用分配的常量值。在程序执行期间不能修改分配的值。

Bjarne Stroustrup 的 explanation简单总结一下:

A class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects.

const 变量必须在类中声明，但不能在其中定义。我们需要在类外定义 const 变量。

T1() : t( 100 ){}

这里的赋值 t = 100 发生在初始化列表中，远在类初始化发生之前。