Question

I’m working on a multiple file project in C++ that gives me linker errors and I don’t understand why. Yes, I have been searching but others all seem to have troubles with third-party libraries and -l link flags, which is not the case here. A small excerpt that should include all relevant pieces of code:

main.cpp

#include "common.h"
//...
void sort_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE], const unsigned int beg = 0, const unsigned int end = TABLE_LENGTH)
{
    //..
}
void precompute_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE])
{
    //..
    #ifdef METHOD_HELLMAN
    precompute_hellman_table(table);
    #endif
    //..
}
int main()
{
    //..
    byte table [TABLE_LENGTH][2][CIPHER_SIZE];
    precompute_table(table);
    //..
}

common.h

#ifndef COMMON_H
#define COMMON_H
//..
typedef unsigned char byte;
//..
#define METHOD_HELLMAN
#define TABLE_LENGTH 40000
#define CIPHER_SIZE 2
//..
void sort_table(byte[TABLE_LENGTH][2][CIPHER_SIZE]);
//..
#endif

hellman.h

#ifndef HELLMAN_H
#define HELLMAN_H

#include "common.h"

extern void precompute_hellman_table(byte[TABLE_LENGTH][2][CIPHER_SIZE]);

#endif

hellman.cpp

#include "hellman.h"
//..
void precompute_hellman_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE])
{
    //..
    sort_table(table);
}

So hellman.cpp uses a general function in main.cpp, which is forward declared in common.h. The following errors are produced when compiling/linking using MinGW:

File                                Message
obj\Release\hellman.o:hellman.cpp   undefined reference to `sort_table(unsigned char (*) [2][2])'

Why is my code incorrect?

Answer 1

Here’s your declaration

// common.h
void sort_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE]);

Here’s your definition

// main.cpp
void sort_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE], 
    const unsigned int beg = 0, const unsigned int end = TABLE_LENGTH)
{
    ...

See the difference?

The declaration and the definition should be the same, except the default values should be in the declaration only.

Like this

// common.h
void sort_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE], 
    const unsigned int beg = 0, const unsigned int end = TABLE_LENGTH);

and this

// main.cpp
void sort_table(byte table[TABLE_LENGTH][2][CIPHER_SIZE], 
    const unsigned int beg, const unsigned int end)
{
    ...