C ++编译器未为未声明的变量引发错误

Question

I tried to search for this specific problem and did not find anythying concrete.

I was using an undeclared variable in my program and the compiler did not complain, it just gave a warning and the program runs fine. My gcc version is 4.1.2

Below is a sample program I wrote to reproduce this, the variable "index" is not declared, why is the compiler treating "index" as a function and where does it find the definition of the function?

#include <iostream>
using namespace std;

int testfunction()
{
     try {
         cout << "inside testfunction method\n";
         return 2;
     } catch(...) {
         cout << "caught exception" << index << endl;
     }
     return 1;
 }
 int main()
 {
     cout << "Testfunction return value : " << testfunction() << endl;
 }

Compiling:

~ g++ throwreturntest.cpp 
throwreturntest.cpp: In function ���int testfunction()���:
throwreturntest.cpp:11: warning: the address of ���char* index(const char*, int)���, will always evaluate as ���true���

Running :

~  ./a.out 
inside testfunction method
Testfunction return value : 2

Answer 1

Looks like index is the name of a GCC builtin function: http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

So it is already declared, just not by you.

Answer 2

The compiler is quite verbose about the situation. It things that index is an address of a function with signature

char *index(const char *s, int c);

See man index(3) . The corresponding header is somewhere in the chain of <iostream>