C++: how to declare non-function without defining it?

Question

Is it possible to declare something like an integer without defining it?

In C++, it is possible to separate the definition and declaration of a function.

// foo.cpp int foo(int); int foo(int a) { return 45; }

But with a non-function it doesn't appear to be

// bar.cpp int bar; int bar = 10;

bar.cpp produces this

$ clang++ -c bar.cpp bar.cpp:2:5: error: redefinition of 'a' int a = 10; ^ bar.cpp:1:5: note: previous definition is here int a; ^ 1 error generated.

Leaving off the type annotation on the second statement produces a different error.

// bar2.cpp int bar; bar = 10;

produces

$ clang++ -c bar2.cpp bar2.cpp:3:1: error: C++ requires a type specifier for all declarations bar = 10; ^ 1 error generated.

Benjamin Lindley · Accepted Answer · 2017-08-08 04:09:25Z

extern int bar; // declares, but does not define bar int bar = 10; // defines bar

Note that this requires bar to have static storage duration. Here's an example usage.

#include <iostream> int main() { extern int bar; std::cout << bar; // this should print 10 } int bar = 10;

I spend an extra five seconds testing to make sure it works, and someone else beats me to it!

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