Deprecated Conversion from String Literal to 'Char*'

C++ deprecated conversion from string constant to 'char*'

This is an error message you see whenever you have a situation like the following:

char* pointer_to_nonconst = "string literal";

Why? Well, C and C++ differ in the type of the string literal. In C the type is array of char and in C++ it is constant array of char. In any case, you are not allowed to change the characters of the string literal, so the const in C++ is not really a restriction but more of a type safety thing. A conversion from const char* to char* is generally not possible without an explicit cast for safety reasons. But for backwards compatibility with C the language C++ still allows assigning a string literal to a char* and gives you a warning about this conversion being deprecated.

So, somewhere you are missing one or more consts in your program for const correctness. But the code you showed to us is not the problem as it does not do this kind of deprecated conversion. The warning must have come from some other place.

How to get rid of `deprecated conversion from string constant to ‘char*’` warnings in GCC

I believe passing -Wno-write-strings to GCC will suppress this warning.

deprecated conversion from string to char *

warning: conversion from a string literal to "char *" is deprecated

This almost certainly means that you are using a C++ compiler. String literals (like "default_file.txt" in your example) was once of type char[] in C and C++ both. Writing to a string literal invokes undefined behavior. Because of this, C++ changed the type of string literals to const char[]. But in C, the type remains char[]. The warning is therefore an indication that you are using a C++ compiler for compiling C code, which is often not wise.

Note that it is however bad practice in C as well as C++, to have a non-const-qualified pointer to a string literal.

Now, I get error: expression must be a modifiable lvalue.

Because you can't assign a value to an array in run-time using assignment operators. The C syntax does not allow this. You would have to use memcpy, strcpy or similar.

Corrected code should be something like this:

static const char* fName = "default_file.txt";

if (argc == 2 ) {
   fName = argv[1];
}

This is fine for C and C++ both, though in C++ it is recommended practice to use std::string instead.

Why is conversion from string constant to 'char*' valid in C but invalid in C++

Up through C++03, your first example was valid, but used a deprecated implicit conversion--a string literal should be treated as being of type char const *, since you can't modify its contents (without causing undefined behavior).

As of C++11, the implicit conversion that had been deprecated was officially removed, so code that depends on it (like your first example) should no longer compile.

You've noted one way to allow the code to compile: although the implicit conversion has been removed, an explicit conversion still works, so you can add a cast. I would not, however, consider this "fixing" the code.

Truly fixing the code requires changing the type of the pointer to the correct type:

char const *p = "abc"; // valid and safe in either C or C++.

As to why it was allowed in C++ (and still is in C): simply because there's a lot of existing code that depends on that implicit conversion, and breaking that code (at least without some official warning) apparently seemed to the standard committees like a bad idea.

Deprecated conversion from string literal to 'char*'

The strings that you enter: "red", "organge" etc are "literal", because they are defined inside the program code itself (they are not read directly from disk, user input /stdin etc.).

This means that if at any point you try to write to your colors you will be directly accessing your original input and thus editing it. This would cause some undesired run-time errors.

Declaring it as a const will make sure that you will never try to write to this pointer and such a run-time error can be avoided.

const char *colors[4] = {"red", "orange", "yellow", "blue"};

If you ever feel like editing these values at runtime, then you should copy the strings first.

deprecated conversion from string constant to 'char*' in c

This should be a warning (though you may have set your compiler to treat warnings as errors, which is often a good idea).

What you want is: char const *p = "hello"; instead.

Attempting to modify a string literal gives undefined behavior. The const prevents you from doing that by accident (i.e., code that attempts to write via the pointer won't compile, unless you remove the const qualifier, such as with a cast).

getting error as deprecated conversion from string constant to 'char*' [-Wwrite-strings]

"" is a c-style string literal whose type is const char[1] and could convert to const char* implicitly. Implicit conversion to char* is not allowed since C++11.

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.

Making initgraph taking const char* is the best. Otherwise you have to perform explicit conversion with const_cast; but note that attempting to modify a string literal results in undefined behavior. If initgraph would do, then you shouldn't pass a string literal.

warning: deprecated conversion from string constant to ‘char*’ when i add a formal parameter in the function def but with no actual parameter

Adding const solves your problem:

void GetHMACCode(unsigned char* buffer,
                 long bufferLength,
                 unsigned char** pResult,
                 unsigned int &nResultLen,
                 const char *extra_arg = "11111111111111111111") // const added here
{}