Am I assigning to the same variable or redefining a new variable?

Question

In A.h:

#ifndef A_h
#define A_h

#include "string"

extern std::string a;

#endif

In A.cpp, in the global scope:

#include "A.h"

std::string a = "a";

If I recall, you shouldn't be using `extern` in `A.cpp`. Just use `std::string a = "a";` — Dave, Aug 11 '13 at 13:03
See http://stackoverflow.com/a/4268620/613130 AND *look at the warnings of your compiler*. — xanatos, Aug 11 '13 at 13:04
@Dave - that's a matter of good style. Even with the `extern`, since there is an initializer, it's a definition. — Pete Becker, Aug 11 '13 at 13:23
@PeteBecker OK, well I guess my comment is still technically correct — the best kind of correct. — Dave, Aug 11 '13 at 13:26
@PeteBecker perfectly clear. I was just being silly; it's a quote. — Dave, Aug 11 '13 at 13:28

score 3 · Accepted Answer · answered Aug 11 '13 at 13:05

3

You are not redefining a variable, because your code has only one definition - the one in the CPP file. The one in the header is a declaration, because it uses an extern keyword. The CPP file provides a definition to the variable declared in the header.

answered Aug 11 '13 at 13:05

Sergey Kalinichenko

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I'm unclear about why you need to declare the type of the variable in the global scope where as you would only need to declare a variable's name to assign to it in a function, can you enlighten me? – Hobbyist Aug 11 '13 at 13:08
@Hobbyist Without the type the variable would not be usable outside of the CPP file where the variable is defined: compiler needs to know the type of `a` in order to let you call its member functions, e.g. `a.size()` or `a.c_str()`. – Sergey Kalinichenko Aug 11 '13 at 13:10
Thanks, having both the type and name looked like a redeclaration to me which confused me. – Hobbyist Aug 11 '13 at 13:15
@Hobbyist You can declare the same thing as many times as you wish, as long as all declarations are consistent with each other. In fact, in C++ a definition is not only a definition, but a combined definition/declaration. It is defining that should generally be done only once to avoid linking errors. – Sergey Kalinichenko Aug 11 '13 at 13:17

score 1 · Answer 2 · answered Aug 11 '13 at 13:14

Think of it this way: The compiler sees A.cpp once, and only once. If your project has many source files that #included A.h, the compiler would see A.h multiple times, once for each #include, as it processes all the *.cpp files. It is logical in that scenario that there be one definition, and multiple declarations

What you have is correct - a variable in global scope in A.cpp. Being global and non-static, it has the potential to be used elsewhere. In fact it is polluting the namespace if it is not used elsewhere.

Am I assigning to the same variable or redefining a new variable?

2 Answers2