C++ How to convert string to char*

c++ casting strtok
Nona Urbiz picture Nona Urbiz · Oct 7, 2009 · Viewed 7.8k times · Source

I need to convert a string to a char * for use in strtok_s and have been unable to figure it out. c_str() converts to a const char *, which is incompatible.

Also, if someone could explain to me why the second strtok_s function (inside the loop) is necessary, it'd be a great help. Why do i need to explicitly advance the token rather than, for example, the while loop it is in, which fetches each line of a file consecutively, implicitly.

while( getline(myFile, line) ) { // Only one line anyway. . . is there a better way?
    char * con = line.c_str();
    token = strtok_s( con, "#", &next_token);
    while ((token != NULL))
    {
        printf( " %s\n", token );
        token = strtok_s( NULL, "#", &next_token);
    }
}

related question.

Answer

Wernsey picture Wernsey · Oct 7, 2009

Use strdup() to copy the const char * returned by c_str() into a char * (remember to free() it afterwards)

Note that strdup() and free() are C, not C++, functions and you'd be better off using methods of std::string instead.

The second strtok_s() is needed because otherwise your loop won't terminate (token's value won't change).

Related questions

Regular cast vs. static_cast vs. dynamic_cast

I've been writing C and C++ code for almost twenty years, but there's one aspect of these languages that I've never really understood. I've obviously used regular casts i.e. MyClass *m = (MyClass *)ptr; all over the place, but there …

Read More
When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?

What are the proper uses of: static_cast dynamic_cast const_cast reinterpret_cast C-style cast (type)value Function-style cast type(value) How does one decide which to use in which specific cases?

Read More
Why use static_cast<int>(x) instead of (int)x?

I've heard that the static_cast function should be preferred to C-style or simple function-style casting. Is this true? Why?

Read More