“const char *”类型的默认参数与“char *”类型的参数不兼容

Question

So I got this code from my teacher but it doesn`t work combined with other code, it works only if it is separatly in a project. The whole code works great, less this part

"Notes" is an other class which works perfectly

class student
{
    char name[30];
    notes marks;
public:
    student(int = 8, char* =" "); //HERE IS WHERE I GOT THE PROBLEM, AT HIS CHAR*
    ~student();
    void read_name();
    void read_marks();
    void modif_mark(int, double);
    void print();
    void check_marks();
};

/*...
  ...
  ...
*here is a lot of code working great*
  ...
  ...
  ...
*/

student::student(int nr_marks, char* sir) :
    marks(nr_marks)
{
    strcpy_s(name, sir);
}

Answer 1

Depending on compiler, C-style string literals may be allocated in readonly memory. Thus they are const char[N+1] ( N is the string length) (which is implicitly convertible to const char* because of array to pointer decay) .

The problem you're having is that it's illegal to drop const qualifiers (with the exception of the infamous const_cast or equivalent C-style cast) .

Since you're only reading from sir , you can fix this by making sir be const char* instead, which doesn't violate const :

class student {
...
student(int = 8, const char* =" "); // problem solved
...
};

student::student(int nr_marks, const char* sir) : // remember to change it here as well
    marks(nr_marks)
{
    strcpy(name, sir);
}

Answer 2

About string literals :

In C, string literals are of type char[] , and can be assigned directly to a (non-const) char* . C++03 allowed it as well (but deprecated it, as literals are const in C++). C++11 no longer allows such assignments without a cast.

Your teacher is possibly more versed in C or "dated" C++. As stated above the language (= modern C++) disallows the assignment / initialization of a char* from a string literal.

Workarounds:

• Use char const * as type for the parameter. That's the reasonable solution when the string is not modified (why would you modify a string literal??)

• Not recommended . When you need to write to that pointer: Store (a copy of) the string literal as a (non const) char[] and reference that. Issues: thread safety; side effects; why would you want to do this??

• Better . If you need to write to / change the string: use std::string .

Answer 3

Below is the error version

print_results("C = ", c);

Below is the Solved Version

print_results((char*)"C = ", c);