How to convert std::string to const char* or char* in C++?

Hello everyone! I just started learning C++ and encountered a problem when working with strings. I have a variable of type `std::string`, and I need to pass it to a function that expects `const char*` or `char*`. For example, I'm trying to open a file using the `fopen()` function, but it doesn't accept `std::string`. Here's my code: ```cpp #include #include #include int main() { std::string filename = "data.txt"; FILE* file = fopen(filename, "r"); // Compilation error! if (file) { std::cout << "File opened successfully" << std::endl; fclose(file); } return 0; } ``` The compiler gives an error: `cannot convert 'std::string' to 'const char*'`. I understand that `std::string` and `const char*` are different types, but how do I convert them?