Declaring large array in C++ header

Question

It's been ages since I last worked with C++.

Situation: I have a large array that needs to be available in different .cpp files. It is immutable, so I thought I better put a const char array[] = … in the header file. But now the array appears several times in the compiled binary, as far as I can see.

What is the proper way to declare large constant arrays in a header, so they won't be compiled into every source object?

There are many duplicates here on SO. If you're too lazy to look for them the solution is to declare it in the header file and define it in a single source file. — Some programmer dude
– Some programmer dude, Commented Feb 28, 2014 at 8:51

abligh · Accepted Answer · 2014-02-28 08:52:05Z

10

If you define an array in a header file (whether or not you initialise it), you will get lots of copies.

You want to declare it in the header:

extern const char array[];

and define it like this in your .c / .cpp file:

const char array[] = ... ;

answered Feb 28, 2014 at 8:52

abligh

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3 Comments

abligh Over a year ago

@Enigma, not it should not be made static. In C, static globals are not visible to other .c files, i.e. the linker will not match one .c files the included extern from the .h file with the definition in the .c file that actually has the array in.

Enigma Over a year ago

My fault: msdn.microsoft.com/en-us/library/s1sb61xd.aspx: "By default, an object or variable that is defined outside all blocks has static duration and external linkage."

James Kanze Over a year ago

Defining a const without initializing it is an error. The important point here is that const objects (unlike other objects) implicitly have internal linkage.

Amit · Accepted Answer · 2014-02-28 08:52:29Z

-2

Use the header guard in header file. So it prevent from multiple declaration. eg.

#ifndef HEADER_H
#define HEADER_H
const char array[] = …
.
.
#endif //HEADER_H

answered Feb 28, 2014 at 8:52

Amit

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2 Comments

Some programmer dude Over a year ago

Header guards only protects against multiple inclusion in the same translation unit, not against inclusion in multiple translation units.

abligh Over a year ago

This is incorrect. As the header is included by each .c / .cpp file, with or without the guard, you will get a copy of the array compiled with each .c / .cpp file.

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