c++ vector with two parameters

Question

I don't quite understand what the following does:

 std::vector<const char*> getRequiredExtensions()
{
    uint32_t glfwExtensionCount = 0;
    const char** glfwExtensions;
    glfwExtensions = glfwGetRequiredInstanceExtensions(&glfwExtensionCount);

    std::vector<const char*> extensions(glfwExtensions, glfwExtensions + glfwExtensionCount);

    if (enableValidationLayers) {
        extensions.push_back(VK_EXT_DEBUG_UTILS_EXTENSION_NAME);
    }

    return extensions;
}

I don't understand this line:

std::vector<const char*> extensions(glfwExtensions, glfwExtensions + glfwExtensionCount);

What exactly does it do? And what does glfwExtensions resolve to, I only get that it's a double pointer (Which is also a new concept for me). And what does the glfwExtensions + glfwExtensionCount mean?

thanks for any help!

Answer 1

Let's concentrate on the following line of code:

std::vector<const char*> extensions(glfwExtensions, glfwExtensions + glfwExtensionCount);

This declares extensions as a variable of the templated std::vector<typename T> type, where the T type resolves to const char* (that is, it declares a vector whose elements are each pointers to constant character data). This vector object is initialized using the constructor that takes two iterator arguments – the form (5) on this cppreference page.

A point of confusion here may be how the arguments are iterators: well, pointers are also iterators (in many ways). In this case, the const char** glfwExtensions; line declares a variable that points to a const char* (which is the type of the vector's elements), and the call to glfwGetRequiredInstanceExtensions makes this point to the beginning of an array of such (const char*) pointers and sets the glfwExtensionCount variable to the number of elements in that array. Further, adding the size of an array to a pointer to its first elements yields a pointer to "one past the end" of the array – which is equivalent to an STL "end" iterator.

Thus, the two arguments to the constructor are, effectively, acting as std::begin(glfwExtensions) and std::end(glfwExtensions) – but those functions can't be used directly, because glfwExtensions is not an array of known size in this context.