Mixing C/C++ using macros and C style strings - c++

Question

gcc/g++ 4.7.2
CXXFLAGS -Wall -Wextra -g -O2
Hello,
I have this header file (mu_test.h) that is written using C style. It contain the following marcos
#define GET_ERROR() ((errno == 0) ? "None" : strerror(errno))
#define LOG_ERR(fmt, ...) fprintf(stderr, "[ERROR] %s:%d: errno: %s " fmt "\n", __func__, __LINE__, GET_ERROR(), ##__VA_ARGS__)
#define MU_ASSERT(test, msg) do { \
if(!(test)) { \
LOG_ERR(msg); \
return msg; \
} \
} while(0)
I have a cpp file (floor_plan_src.cpp) compiled using g++ that includes the mu_test.h
#include "mu_test.h"
char* test_memory_allocation()
{
plan = new floor_plan();
MU_ASSERT(plan != NULL, "Failed to allocate memory for floor_plan");
return NULL;
}
I get this warning:
deprecated conversion from string constant to ‘char*’
So my string constant that I am passing to that function-like marco doesn't like it (C strings) as I have compiled my source using g++.
I thought this problem was to do with mixing c/c++.
solution 1: wrap all the macros in the mu_test.h with extern "C"
#ifdef __cplusplus
extern "C"
{
#endif /* _cplusplus */
#define GET_ERROR() ((errno == 0) ? "None" : strerror(errno))
#define LOG_ERR(fmt, ...) fprintf(stderr, "[ERROR] %s:%d: errno: %s " fmt "\n", __func__, __LINE__, GET_ERROR(), ##__VA_ARGS__)
#define MU_ASSERT(test, msg) do { \
if(!(test)) { \
LOG_ERR(msg); \
return msg; \
} \
} while(0)
#ifdef __cplusplus
}
#endif /* __cplusplus */
Solution 1 still gave me the same warning.
solution 2: wrap the header file in floor_plan_src.cpp
extern "C" {
#include "mu_test.h"
}
Solution 2 still gave me the same warning
Solution 3: wrap the function
extern "C" char* test_memory_allocation()
{
plan = new floor_plan();
MU_ASSERT(plan != NULL, "Failed to allocate memory for floor_plan");
return NULL;
}
Solution 3 same as above
Solution 4:
Try and convert the constant string to a non-const char*
MU_ASSERT(plan != NULL, (char*)"Failed to allocate memory for floor_plan");
gave the following error:
expected primary-expression before char
"[ERROR] %s:%d: errno: %s " cannot be used as a function
Many thanks for any suggestions,

Answer 1

The problem is that your test_memory_allocation might return a string literal, and you shouldn't make a string literal decay to non-const char*: it's allowed in C++, but still deprecated.
Your code gets expanded in:
char* test_memory_allocation()
{
plan = new floor_plan();
do {
if(!(plan != NULL)) {
LOG_ERR("Failed to allocate memory for floor_plan")
return "Failed to allocate memory for floor_plan";
}
} while(0);
return NULL;
}
To fix it you just need to make test_memory_allocation return a const char* or otherwise you could return a pointer to something that could decay to a non-const char* (for example a static char array or to a heap-allocated memory area).
extern "C" is only needed to avoid C++ name mangling, it only affects functions, not macros.