for C++ programmers

libcurl provides a C API. C and C++ are similar but not the same. There are a few things to keep in mind when using libcurl in C++.

Strings are C strings, not C++ string objects

When you pass strings to libcurl’s APIs that accept char * that means you cannot pass in C++ strings or objects to those functions.

For example, if you build a string with C++ and then want that string used as a URL:

std::string url = "https://example.com/foo.asp?name=" + i;
curl_easy_setopt(curl, CURLOPT_URL, url.c_str());

Callback considerations

libcurl is written in C and is unaware of C++ class instances or member functions, which require an implicit this pointer. You can work around this by using a static member function (or a standalone C-style function) as the callback and passing the object instance via the user data pointer you set with libcurl (for example, CURLOPT_WRITEDATA for the write callback). Inside the static callback, cast that void * back to your class type to invoke non-static member functions.

Here’s an example of a write callback using a C++ method as callback:

// f is the pointer to your object.
static size_t YourClass::func(void *buffer, size_t sz,
                              size_t n, void *f)
{
  // Call non-static member function.
  static_cast<YourClass*>(f)->nonStaticFunction();
}

// This is how you pass pointer to the static function:
curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func);
curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);