在头文件中使用`const char thing [] =“foo”;`

Question

I want to create a constant global char array such that

• It can be used in multiple translation units.
• The length of the array is deduced from the string literal used to initialize it.
• That string literal exist exactly once in my source (and if possible the object files).
• Where all the translation unit have compile time access to the length.
• Has no ODR violations when the multiple translation units are linked together.

In theory this should be possible by using const char[] and putting the deceleration/definition in the header file in a way that forces the data/symbol into a COMDAT section, but I don't know if the standard (or even any compilers) support that.

ps Assume whatever idiom is used will be used for hundreds to thousands of constants across many files.

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Edit: the "cleanest" solution I know of that give all the points is:

template<bool> struct data_ {
  static const char kFoo[];
};

template<> const char data_<true>::kFoo[] = "bar\0other\0stuff\0";
typedef data_<true> data;


#include <stdio.h>

template<typename T, int N>
void Print(T(&var)[N]) { printf("%d %s\n", N, var); }

int main() { Print(data::kFoo); return 0; }

Which is still rather ugly.

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OTOH If I just throw out 3b (guaranteed same storage module inlineing) then this work:

const char kFoo[] = "bar\0other\0stuff\0";

because it's has internal linkage by default. A good linker could merge these, but at that point you can't say anything about the the relationship between the equality of the addresses/identifiers (ie don't cast it to a pointer and use that as an identity). But that's a caveat that's likely to be benign almost all of the time.

Answer 1

Header:

#define LITERAL "Hello, world"
extern char const literal[sizeof LITERAL];

One source file:

char const literal[] = LITERAL;

There's still no guarantee that any particular compiler/linker only make one copy of the string literal (but it does guarantee the requirement that &literal[0] is the same in all units).

Answer 2

Since C++17 you can write in the header:

inline char const thing[] = "foo";

which meets all the criteria.

Note: inline variables have external linkage unless explicitly declared as static . The rule about const variables defaulting to internal linkage only applies to non-inline variables.

Answer 3

你不想要

const char * const LITERAL="foo";