C++

curl with Variable Substitution in a JSON Request Body

See more CURL Examples

This example shows how to use variables inside a JSON request body using the {{variable_name}} syntax. When the HTTP request’s Content-Type indicates JSON, Chilkat automatically applies proper JSON escaping to each substituted value, ensuring the resulting JSON remains valid. C++

#include <CkStringBuilder.h>
#include <CkHttpCurl.h>

void ChilkatSample(void)
    {
    bool success = false;

    // Variable names are enclosed between {{ and }}
    // Important: The variable {{var_name}} should be placed inside the quotes.

    // This is correct:
    // curl -X POST https://api.example.com/messages \
    //   -H "Content-Type: application/json" \
    //   -d '{"text":"{{message}}"}'

    // This is incorrect:
    //   -d '{"text":{{message}}}'

    CkStringBuilder sbCurl;
    sbCurl.AppendLn("curl -X POST https://api.example.com/messages \\");
    sbCurl.AppendLn("  -H \"Content-Type: application/json\" \\");
    sbCurl.AppendLn("  -d '{\"text\":\"{{message}}\"}'");

    CkHttpCurl curl;

    // In this example, the value we'll provide for the "message" variable
    // will contain chars that require JSON escaping.
    curl.SetVar("message","He said \"Hello, world!\"");

    // To demonstrate how the variables are replaced, this example does not execute the curl command. 
    // Instead, it generates the raw HTTP request that would be sent if the curl command were run.
    CkStringBuilder sbRawRequest;
    success = curl.ToRawRequest(sbCurl.getAsString(),sbRawRequest);
    if (success == false) {
        std::cout << curl.lastErrorText() << "\r\n";
        return;
    }

    std::cout << sbRawRequest.getAsString() << "\r\n";

    // The output is shown below.
    // Notice that the quote chars around "Hello World!" are properly JSON escaped.

    // POST /messages HTTP/1.1
    // Host: api.example.com
    // Content-Type: application/json
    // Content-Length: 36
    // 
    // {"text":"He said \"Hello, world!\""}
    }