Can I use libcurls CURLOPT_WRITEFUNCTION with a C++11 lambda expression?

I tried to use a C++11 lambda expression with CURLOPT_WRITEFUNCTION, but the program crashes at runtime with an access violation. I'm not sure how to look further into this due to lack of C++11 knowledge, but maybe someone else has an idea how to make this work.

The function:

#ifndef CURL_GET_H
#define CURL_GET_H

#include <curl/curl.h>
#include <curl/easy.h>
#include <vector>
#include <s开发者_如何学编程tring>

std::vector<std::string> curl_get(const char* url)
{
    CURL *curl;
    CURLcode res;

    std::vector<std::string> content;
    auto curl_callback = [](void *ptr, size_t size, size_t nmemb, void *stream) -> size_t {
        // does nothing at the moment due to testing...
        return size * nmemb;
    };

    curl = curl_easy_init();
    if (curl)
    {
        curl_easy_setopt(curl, CURLOPT_URL, "http://localhost/aaa.txt");
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_callback);
        res = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
    }

    return content;
}

#endif // CURL_GET_H

The error:

Unbehandelte Ausnahme bei 0x000000cc in lpip_dl.exe: 0xC0000005: Zugriffsverletzung bei Position 0x00000000000000cc.

(Access violation at position 0x00000000000000cc)

Happens when curl wants to use the callback:

wrote = data->set.fwrite_func(ptr, 1, len, data->set.out);

You actually can do that by casting the lambda function to function pointer. You can first make a typedef to make cast easier.

typedef size_t(*CURL_WRITEFUNCTION_PTR)(void*, size_t, size_t, void*);

Then you use static_cast.

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, static_cast<CURL_WRITEFUNCTION_PTR>(curl_callback));

Note: In order to convert to C function pointer, you can only use empty captures [].

This can be done with the + prefix, which returns a C-style function pointer instead. But this only works with stateless lambdas (empty capture list i.e. []).

auto lambda = +[](void *ptr, size_t size, size_t nmemb, void *stream) -> size_t {
    // does nothing at the moment due to testing...
    return size * nmemb;
};

or

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, +[](void *ptr, size_t size, size_t nmemb, void *stream) -> size_t {
    // does nothing at the moment due to testing...
    return size * nmemb;
});

libcurl is plain C library, you need to set a callback that can be called from a such. This means funny C++ things need to be "C'ified" first to work. Like into an old-style function pointer.

This is also addressed in the libcurl FAQ entry "Using C++ non-static functions for callbacks?"

See also: C-style Callbacks in C++11

Casting the lambda to the proper function pointer works for me in Linux:

auto curl_callback = static_cast<size_t(*)(char*,size_t,size_t,void*)>([](...){...});