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如何在C++头文件中定义const成员变量？
如何在C++头文件中定义const成员变量？
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一、必须在构造函数中初始化const成员变量。
frank@userver:~/project/test/const-test/const4_cpp$
frank@userver:~/project/test/const-test/const4_cpp$ cat const.h -n
1 #ifndef CONST_H
2 #define CONST_H
3
4 #include<iostream>
5 using std::cout;
6 using std::endl;
7
8 class TestType{
9 const int m_const;
10 public:
11 TestType(int dat = 0) {}
12 void Show()const{
13 cout << "this=" << this << ", ";
14 cout << "m_const:(" << &m_const << ", " << m_const << ")\n";
15 }
16 };
17
18 #endif // #ifndef CONST_H
frank@userver:~/project/test/const-test/const4_cpp$ cat main.cpp -n
1 #include "const.h"
2
3 int main(){
4 cout << __func__ << endl;
5 TestType obj(4);
6 obj.Show();
7 }
frank@userver:~/project/test/const-test/const4_cpp$ g++ main.cpp
In file included from main.cpp:1:0:
const.h: In constructor ‘TestType::TestType(int)’:
const.h:11:5: error: uninitialized member ‘TestType::m_const’ with ‘const’ type ‘const int’ [-fpermissive]
TestType(int dat = 0) {}
^
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二、不能在构造函数体中初始化const成员变量。
frank@userver:~/project/test/const-test/const4_cpp$ cat const.h -n
1 #ifndef CONST_H
2 #define CONST_H
3
4 #include<iostream>
5 using std::cout;
6 using std::endl;
7
8 class TestType{
9 const int m_const;
10 public:
11 TestType(int dat = 0) { m_const = dat; }
12 void Show()const{
13 cout << "this=" << this << ", ";
14 cout << "m_const:(" << &m_const << ", " << m_const << ")\n";
15 }
16 };
17
18 #endif // #ifndef CONST_H
frank@userver:~/project/test/const-test/const4_cpp$ cat main.cpp -n
1 #include "const.h"
2
3 int main(){
4 cout << __func__ << endl;
5 TestType obj(4);
6 obj.Show();
7 }
frank@userver:~/project/test/const-test/const4_cpp$ g++ main.cpp
In file included from main.cpp:1:0:
const.h: In constructor ‘TestType::TestType(int)’:
const.h:11:5: error: uninitialized member ‘TestType::m_const’ with ‘const’ type ‘const int’ [-fpermissive]
TestType(int dat = 0) { m_const = dat; }
^
const.h:11:37: error: assignment of read-only member ‘TestType::m_const’
TestType(int dat = 0) { m_const = dat; }
^
frank@userver:~/project/test/const-test/const4_cpp$
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三、只能在构造函数初始化列表中初始化const成员变量。每个对象都有一个m_const。
frank@userver:~/project/test/const-test/const4_cpp$ cat const.h -n
1 #ifndef CONST_H
2 #define CONST_H
3
4 #include<iostream>
5 using std::cout;
6 using std::endl;
7
8 class TestType{
9 const int m_const;
10 public:
11 TestType(int dat = 0) : m_const(dat) {}
12 void Show()const{
13 cout << "this=" << this << ", ";
14 cout << "m_const:(" << &m_const << ", " << m_const << ")\n";
15 }
16 };
17
18 #endif // #ifndef CONST_H
frank@userver:~/project/test/const-test/const4_cpp$ cat main.cpp -n
1 #include "const.h"
2
3 int main(){
4 cout << __func__ << endl;
5 TestType obj(4);
6 obj.Show();
7 }
frank@userver:~/project/test/const-test/const4_cpp$ g++ main.cpp
frank@userver:~/project/test/const-test/const4_cpp$ ./a.out
main
this=0x7fff7ab7d960, m_const:(0x7fff7ab7d960, 4)
frank@userver:~/project/test/const-test/const4_cpp$
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本文出自 “用C++写诗” 博客，谢绝转载！
如何在C++头文件中定义const成员变量？