strdup pointer version needs a temporary pointer

Question

I'm implementing a strdup function as an exercise.

char* strdupPtr(const char* str) {
    const size_t sz{strlen(str)+1};
    char *save, *temp;
    save = temp = (char*)malloc(sz);
    while((*temp++ = *str++));  // compiler warning with only 1 set of   parenthesis
    return save;
}

After a few failures I found out that it works properly when the "save"(wiki reference) pointer is returned but not when "temp" is returned. Why do I need to return save instead of temp directly when dealing with pointers(a subscript array version works without the use of save)?

Answer 1

Within the function pointer temp is incremented.

while((*temp++ = *str++));  

So if to return temp then it will not contain the start address of the allocated memory.

For example these use cases will be invalid.

char *p = strdup( "Hello World" );

puts( p );

free( p );

Take into account that in C++ it is better to use operator new. For example

char * strdupPtr( const char *s ) 
{
    char *p = new char[std::strlen( s ) + 1];

    std::strcpy( p, s );

    return p;
}

Or you could even write

char * strdupPtr( const char *s ) 
{
    char *p = new char[std::strlen( s ) + 1];

    return std::strcpy( p, s );
}