Joining a variable inside a JSON formatted post request C++ - c++

Question

So I have this put request which submits to a service running on localhost.
Doing it the way below works just fine, note I have replaced the actual acc name with ACC_NAME and password with ACC_PASSWORD.
curl_easy_setopt(curl_account_login, CURLOPT_POSTFIELDS, "{\r\n \"username\": \ACC_NAME\,\r\n \"password\": \"ACC_PASSWORD\,\r\n \"persistLogin\": false\r\n}\r\n");
However when I wanted to pass in a variable containing the acc_name and acc_password, it does not work, I get an error response from server.
The below request is using the variable joined inside the JSON string, which gives me the error response.
curl_easy_setopt(curl_account_login, CURLOPT_POSTFIELDS, "{\r\n \"username\": \""+acc_name+"\,\r\n \"password\": \""+acc_password+"\,\r\n \"persistLogin\": false\r\n}\r\n");
I can't figure out what I am doing wrong when I am joining the string variable into the request.
It works just fine by plain, if I write the account credentials directly into the request and not in a variable.
Regards

Answer 1

C-style string literals (like "something") are of type const char [] (this is a null-terminated character array), which decays into a const char* (a pointer). Thus, + (as in "something" + "another") is just adding two addresses, resulting in an invalid pointer value. You cannot concatenate C-style strings by simply using + that way.
Assuming you are using C++ (not C), as indicated by your question being tagged c++, I suggest C++ string objects instead, this will allow easy concatenation.
Also, there's a note in curl documentation about CURLOPT_POSTFIELDS, mentioning that the data pointed to is not copied, thus requiring you to make sure the pointer remains valid until the associated transfer finishes. Because of that, I would prefer using CURLOPT_COPYPOSTFIELDS instead.
To sum up, do something like this:
#include <string>
// ...
std::string postfields
{
std::string{ "{" }
+ R"("username":")" + ACC_NAME
+ R"(","password":")" + ACC_PASSWORD
+ R"(","persistLogin":false})"
};
curl_easy_setopt(curl_account_login, CURLOPT_COPYPOSTFIELDS, postfields.data());