Fixing the Warning: Deprecated Conversion from String Constant to 'char*' [-Wwrite-strings] - A Step-by-Step Guide

In this guide, we will explore the warning message "Deprecated conversion from string constant to 'char*'", why it occurs, and how to fix it in your C++ code. This warning is generated by the -Wwrite-strings option in GCC, and it's related to the type of string literals in C++.

Table of Contents

Understanding the Warning

In C++, string literals like "hello" are of the type const char [], which means they are constant character arrays. However, it's a common mistake to assign them to non-const character pointers (char*). This practice is deprecated and can lead to undefined behavior.

Here's an example of code that generates this warning:

int main() {
    char* str = "hello";
    return 0;

In this code, we're trying to assign a string literal (constant) to a non-const character pointer, which can cause issues if we try to modify the string later in the program.

Step-by-Step Guide to Fix the Warning

To fix this warning, follow these steps:

Identify the line of code generating the warning.

Check if the string literal is being assigned to a non-const character pointer, like char*. If so, change the pointer type to const char* or const char[].

If you need to modify the string later in your program, consider using a different data type, like std::string or std::vector<char>.

Let's see an example of fixing the warning in the code we discussed earlier:

int main() {
    const char* str = "hello"; // Changed 'char*' to 'const char*'
    return 0;

By changing the pointer type to const char*, we have fixed the warning and ensured that we won't accidentally modify the string literal.


Why is it dangerous to assign string literals to non-const pointers?

Assigning string literals to non-const pointers can lead to undefined behavior if the program attempts to modify the string. String literals are stored in read-only memory, and any attempt to modify them can result in a segmentation fault or other unwanted consequences.

What is the type of a string literal in C++?

In C++, string literals are of the type const char[], which means they are constant character arrays. They cannot be modified and should be assigned to const pointers or arrays.

Can I use std::string instead of char*?

Yes, you can use the std::string class from the C++ Standard Library to store and manipulate strings. The std::string class automatically manages the memory and provides several useful methods for working with strings.

What is the difference between char* and const char*?

char* is a pointer to a character or a character array, which can be modified. const char*, on the other hand, is a pointer to a constant character or a constant character array, which cannot be modified.

How can I convert a string literal to a modifiable character array?

If you need to create a modifiable copy of a string literal, you can use the strcpy() function from the <cstring> header, or you can use the std::string class or std::vector<char> to store and manipulate the string.

Example using strcpy():

#include <cstring>

int main() {
    const char* str_literal = "hello";
    char str_modifiable[6];
    strcpy(str_modifiable, str_literal);
    return 0;

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