C/C++中define的使用

#include <iostream>

using namespace std;

#define a 10

void foo();
void bar();

void foo(){
#undef a
#define a 50
}

int main(){

cout<<"main "<<a<<endl;
foo();
cout<<"main "<<a<<endl;
bar();
cout<<"main "<<a<<endl;

return 0;
}

void bar(){
#undef a
#define a 100
cout<<"bar "<<a<<endl;
}

main 50
main 50
bar 100
main 50