How do I define a const member variable in a C + + header file?

How do I define a const member variable in a C + + header file?

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A const member variable must be initialized in the constructor.

[Email protected]:~/project/test/const-test/const4_cpp$

[Email protected]:~/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;

Public:

Testtype (int dat = 0) {}

void Show () const{

cout << "this=" << this << ",";

cout << "M_const: (" << &m_const << "," << m_const << ") \ n";

15}

16};

17

#endif//#ifndef Const_h

[Email protected]:~/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}

[Email protected]:~/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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Second, the const member variable cannot be initialized in the constructor body.

[Email protected]:~/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;

Public:

Testtype (int dat = 0) {m_const = dat;}

void Show () const{

cout << "this=" << this << ",";

cout << "M_const: (" << &m_const << "," << m_const << ") \ n";

15}

16};

17

#endif//#ifndef Const_h

[Email protected]:~/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}

[Email protected]:~/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;}

^

[Email protected]:~/project/test/const-test/const4_cpp$

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The const member variable can only be initialized in the constructor initialization list. Each object has a m_const.

[Email protected]:~/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;

Public:

Testtype (int dat = 0): m_const (DAT) {}

void Show () const{

cout << "this=" << this << ",";

cout << "M_const: (" << &m_const << "," << m_const << ") \ n";

15}

16};

17

#endif//#ifndef Const_h

[Email protected]:~/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}

[Email protected]:~/project/test/const-test/const4_cpp$ g++ main.cpp

[Email protected]:~/project/test/const-test/const4_cpp$./a.out

Main

this=0x7fff7ab7d960, M_const: (0x7fff7ab7d960, 4)

[Email protected]:~/project/test/const-test/const4_cpp$

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This article from the "write poetry with C + +" blog, declined reprint!

How do I define a const member variable in a C + + header file?