poetryofruins poetryofruins - 1 month ago 14
C++ Question

glGetShaderInfoLog to std::string?

This is a snippet from OpenGL Super Bible 7th edition:

GLint log_length;
glGetShaderiv(fs, GL_INFO_LOG_LENGTH, &log_length);
std::string str;
str.reserve(log_length);
glGetShaderInfoLog(fs, log_length, NULL, str.c_str());


At a first glance it seemed weird to me to pass
str.c_str()
as an argument to this function, and of course clang immediatelly protested:
cannot initialize a parameter of type 'GLchar *' (aka 'char *') with an rvalue of type 'const char *'
.

So I tried to investigate and changed
str.c_str()
to
str.data()
, which should provide a pointer to the internal data array, but this produces the same error message. Edit:
c_str()
and
data()
are actually the same thing (in c++11 at least), so it doesn't matter which one we use.

I did some searches, but didn't find a solution (although I'm quite new to C++ and some things are still not obvious to me).

Is this an error in the book? And is there a way to make it work with std::string?

Asu Asu
Answer

Both string::c_str() and string::data() return a const char* until C++17. Since C++17, string::data() has an overload that will return a char*. This means this code snippet is invalid.

Edit: An easier and as efficient solution is to use &str[0]. The subscript operator will return a char&.

If you cannot use C++17, you could use a std::vector<char>, which does have a non-const data(), but this will cause a copy when constructing a std::string:

// ...
std::vector<char> v(log_length);
glGetShaderInfoLog(fs, log_length, NULL, v.data());
std::string s(begin(v), end(v));

Also, reserve() isn't sufficient, because the actual size() is not changed. If c_str() returned a char*, the snippet would actually also cause undefined behavior.