能否在头文件中定义全局const变量？
一、在没有类定义的头文件中定义全局const变量g_const。
用gcc编译错误，因为重定义。
用g++可以编译。但运行结果显示，在test.c和main.c中，变量的值相同，但地址不同，说明是两个变量。
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frank@userver:~/project/test/const-test/const1_c$ cat const.h
#ifndef CONST_H
#define CONST_H
const int g_const = 12;
#endif // #ifndef CONST_H
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frank@userver:~/project/test/const-test/const1_c$ cat test.h
#ifndef TEST_H
#define TEST_H
void test();
#endif // #ifndef TEST_H
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frank@userver:~/project/test/const-test/const1_c$ cat test.c
#include <stdio.h>
#include "const.h"
#include "test.h"
void test(){
printf("[%s] g_const(%p,%u)\n", __FILE__, &g_const, g_const);
}
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frank@userver:~/project/test/const-test/const1_c$ cat main.c
#include <stdio.h>
#include "const.h"
#include "test.h"
int main(){
printf("[%s] g_const(%p,%u)\n", __FILE__, &g_const, g_const);
test();
return 0;
}
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frank@userver:~/project/test/const-test/const1_c$ gcc main.c test.c
/tmp/ccOM9phK.o:(.rodata+0x0): multiple definition of `g_const'
/tmp/cciYjaBU.o:(.rodata+0x0): first defined here
collect2: error: ld returned 1 exit status
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frank@userver:~/project/test/const-test/const1_c$ g++ main.c test.c
frank@userver:~/project/test/const-test/const1_c$ ./a.out
[main.c] g_const(0x40061c,12)
[test.c] g_const(0x400638,12)
frank@userver:~/project/test/const-test/const1_c$
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二、在有类定义的头文件中定义全局const变量。
用g++可以编译。
在类的成员函数中，g_const的地址保持一致，说明在类的不同的对象中会访问到同一个全局const变量。
在类外部的函数中，运行结果与编译顺序有关。
在先编译的文件的函数中，g_const地址与类的成员函数一致，说明与类内部的g_const是同一个变量。。
但是在后编译的文件的函数中，g_const的地址不同，说明是另一个变量。
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frank@userver:~/project/test/const-test/const3_cpp$ cat const.h
#ifndef CONST_H
#define CONST_H
#include<iostream>
using std::cout;
using std::endl;
const int g_const = 10;
class TestType{
public:
TestType(){}
void Show()const{
cout << "this=" << this << ", ";
cout << "g_const:(" << &g_const << ", " << g_const << ")\n";
}
};
#endif // #ifndef CONST_H
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frank@userver:~/project/test/const-test/const3_cpp$ cat test.h
#ifndef TEST_H
#define TEST_H
void test();
#endif // #ifndef TEST_H
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frank@userver:~/project/test/const-test/const3_cpp$ cat test.cpp
#include "test.h"
#include "const.h"
void test(){
cout << __func__ << endl;
cout << "g_const = (" << &g_const << ", " << g_const << ")\n";
TestType obj;
obj.Show();
}
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frank@userver:~/project/test/const-test/const3_cpp$ cat main.cpp
#include "test.h"
#include "const.h"
int main(){
cout << __func__ << endl;
cout << "g_const: (" << &g_const << ", " << g_const << ")\n";
TestType obj;
obj.Show();
test();
}
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frank@userver:~/project/test/const-test/const3_cpp$ g++ main.cpp test.cpp
frank@userver:~/project/test/const-test/const3_cpp$ ./a.out
main
g_const: (0x400bd8, 10)
this=0x7fffcddbd39f, g_const:(0x400bd8, 10)
test
g_const = (0x400c08, 10)
this=0x7fffcddbd37f, g_const:(0x400bd8, 10)
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frank@userver:~/project/test/const-test/const3_cpp$ g++ test.cpp main.cpp
frank@userver:~/project/test/const-test/const3_cpp$ ./a.out
main
g_const: (0x400c08, 10)
this=0x7fff46b4525f, g_const:(0x400bd8, 10)
test
g_const = (0x400bd8, 10)
this=0x7fff46b4523f, g_const:(0x400bd8, 10)
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三、结论：不要在头文件中定义全局const变量。
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