How to pass an std::vector<char>* as void* then cast the void* to std::vector<char>* back again?

Question

I am using esp-idf to do some http requests, and I want to use c++ because I am more comfortable with it. The esp_http_client works more or less like this:

1. You populate a designated initializer:

char[some_length] local_response_buffer;

// I want to use std::vector<char> local_response_buffer for dynamic allocation

esp_http_client_config_t config = {
        .host = "httpbin.org",
        .path = "/get",
        .disable_auto_redirect = true,
        .event_handler = http_event_handler,
        .user_data = local_response_buffer, // user_data is void* so the type is erased
    };

2. While writing the event handler when data is received you get back the void* user_data and copy a temporary buffer of http data chunk into it:

esp_err_t http_event_handler(esp_http_client_event_t *evt)
{
    switch (evt->event_id) {
    /* handle other cases... */
    case HTTP_EVENT_ON_DATA:
        // append evt->data (which is actually C-style char* casted to void*) 
        // into evt->user_data (this is our user_data that has been casted to void*).
        // I want to use std::vector::insert here for this purpose.
        break;
}

I want to use std::vector instead of char[] for convenient dynamic allocation. What is the best way of doing this?

Answer 1

You already stated the answer in your question title. You can safely cast a std::vector* pointer to a void* pointer, and then cast it back to a std::vector* pointer.

You can use the & operator to get the address of the vector, eg:

vector<char> local_response_buffer;

esp_http_client_config_t config = {
    ...
    .user_data = &local_response_buffer
};

...

esp_err_t http_event_handler(esp_http_client_event_t *evt)
{
    ...
    vector<char> *buffer = static_cast<vector<char>*>(evt->user_data);
    char *data = static_cast<char*>(evt->data);
    buffer->insert(buffer->end(), data, data + evt->data_len);
    ...
}

Alternatively, you might consider using std::string instead of std::vector, eg:

string local_response_buffer;

esp_http_client_config_t config = {
    ...
    .user_data = &local_response_buffer
};

...

esp_err_t http_event_handler(esp_http_client_event_t *evt)
{
    ...
    string *buffer = static_cast<string*>(evt->user_data);
    buffer->append(static_cast<char*>(evt->data), evt->data_len);
    ...
}