A module problem

[bashill.zhu]

Administrator@HILL /d/Users/Administrator/Desktop/tcplex/ch8/ex3_test
$ cat lexer.h
namespace Lexer
{
char get_char();
void foo();
}
$ cat lexer.cpp
#include "lexer.h"
#include <iostream>
namespace Lexer
{
extern std::istream* input;
int g_i;
}
char Lexer::get_char()
{
char ch;
g_i = 100;
input->get(ch);
return ch;
}

void Lexer::foo()
{
g_i = 200;
char c;
input->get(c);
return;
}
$ cat main.cpp
#include "lexer.h"
#include <iostream>
using namespace std;
istream* input;
int main()
{
input = &cin;
char ch = Lexer::get_char();
}
g++ -g -Wall -c lexer.cpp
g++ -g -Wall -c main.cpp
main.cpp: In function `int main()':
main.cpp:8: warning: unused variable 'ch'
g++ -o test.exe lexer.o main.o
lexer.o: In function `ZN5Lexer8get_charEv':
d:/Users/Administrator/Desktop/tcplex/ch8/ex3_test/lexer.cpp:12:
undefined reference to `Lexer::input'
lexer.o: In function `ZN5Lexer3fooEv':
d:/Users/Administrator/Desktop/tcplex/ch8/ex3_test/lexer.cpp:20:
undefined reference to `Lexer::input'
collect2: ld returned 1 exit status
make: *** [test.exe] Error 1


Could some one give me some help about this issue?

 

[Rolf Magnus]

namespace Lexer
{
extern std::istream* input;
int g_i;
}

#include "lexer.h"
#include <iostream>
using namespace std;
istream* input;
int main()
{
input = &cin;
char ch = Lexer::get_char();
}

undefined reference to `Lexer::input'
undefined reference to `Lexer::input'

Could some one give me some help about this issue?

Well, the linker is right. There is no definition of an object named input
in namespace Lexer. There is only one in the 'global' namespace.

 

[bashill.zhu]

Well, the linker is right. There is no definition of an object named input
in namespace Lexer. There is only one in the 'global' namespace.

Ok I got the point . Thanks a lot .
When we declare a varible , we need care its namespace limitation.

 

[bashill.zhu]

namespace Lexer {

}

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I got the point.
When do this i should initialize the input pointer oursize the module
to let it points to a object .
Thanks in advance.