CODEWITHSUNDEEP
c++
jdhabez
jdhabez

Reputation: 41

C Integer losing its constant status when being used in C++ code

I have a C/C++ mixed project that I am currently working on. I have a global constants C code that I want to be able to access from my C++ scripts and C scripts. In this particular situation I am trying to use a C++ array with variable dimensions made up of constant integers defined in the C global constants code. However, when I try to use or declare this array I get the error that the array dimensions are not integer constants (though I defined them to be integer constants in my C code).

constants.c

const int x = 5;

constants.h

#ifdef __cplusplus
extern "C" {
#endif

extern const int x;

#ifdef __cplusplus
}
#endif

my_cpp.h

#include "constants.h"

my_cpp.cpp

#include "my_cpp.h"

double A[x];

So here, I would get an error stating that x is not an integer constant. Where did I go wrong?

Upvotes: 1

Views: 97

Answers (1)

SergeyA
SergeyA

Reputation: 62593

You didn't put definition of x in the header file, so it is not a core constant expression.

Easiest way to fix would be to use const int x = 5; in your header file. Alternatively, you can use enum: enum { x = 5; } - this gives you true prvalue, pretty much like the literal 5 itself.

More information on what is a constant expression in C++ (and array indexes need to be constant expressions in C++) can be found here: https://en.cppreference.com/w/cpp/language/constant_expression

Upvotes: 7

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