How to Post Messages to Slack using C++ (Win32 API + WinHTTP) [Webhook Supported]

I want to post messages to Slack from C++. It’s common in Node.js or Python, but doing it with “C++ × Win32 API × WinHTTP” is quite rare, isn’t it?

In this article, I will explain how to send messages from C++ to Slack using a Webhook URL, step-by-step and in an easy-to-understand manner.

✅ Overall Flow

To post to Slack, follow these steps:

Obtain a Slack Webhook URL (API key)
Send a POST request using WinHTTP
Assemble the message body in JSON format
Check the result and you’re done!

🔑 Step 1: How to Obtain a Slack Webhook URL

Slack allows you to easily post messages from external services using a feature called Incoming Webhooks.

Steps to Obtain

Access https://api.slack.com/apps
Click Create New App
Choose From scratch, then select the app name and the workspace to post to
Select “Incoming Webhooks” from the left menu and enable it
Click “Add New Webhook to Workspace” and select a channel
Copy the generated URL (e.g., https://hooks.slack.com/services/xxx/yyy/zzz)

This URL functions like an API key.

💻 Step 2: Send a Message to Slack with C++ Code

Technologies Used

Win32 API
WinHTTP (Standard Library)
JSON formatted messages

Sample Code (Posting to Slack)

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#include <windows.h>
#include <winhttp.h>
#include <iostream>

#pragma comment(lib, "winhttp.lib")

bool PostToSlack(const std::wstring& webhookUrl, const std::string& messageJson) {
    // Parse URL
    URL_COMPONENTS urlComp{};
    wchar_t hostName[256];
    wchar_t urlPath[1024];

    urlComp.dwStructSize = sizeof(urlComp);
    urlComp.lpszHostName = hostName;
    urlComp.dwHostNameLength = _countof(hostName);
    urlComp.lpszUrlPath = urlPath;
    urlComp.dwUrlPathLength = _countof(urlPath);

    if (!WinHttpCrackUrl(webhookUrl.c_str(), 0, 0, &urlComp)) {
        std::wcerr << L"Failed to parse URL\n";
        return false;
    }

    // Connect and start HTTP session
    HINTERNET hSession = WinHttpOpen(L"SlackPoster/1.0",
                                     WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                                     WINHTTP_NO_PROXY_NAME,
                                     WINHTTP_NO_PROXY_BYPASS, 0);
    HINTERNET hConnect = WinHttpConnect(hSession, hostName, urlComp.nPort, 0);
    HINTERNET hRequest = WinHttpOpenRequest(hConnect, L"POST", urlPath,
                                            NULL, WINHTTP_NO_REFERER,
                                            WINHTTP_DEFAULT_ACCEPT_TYPES,
                                            WINHTTP_FLAG_SECURE);

    std::wstring headers = L"Content-Type: application/json\r\n";
    BOOL result = WinHttpSendRequest(hRequest,
                                     headers.c_str(),
                                     -1,
                                     (LPVOID)messageJson.c_str(),
                                     messageJson.length(),
                                     messageJson.length(),
                                     0);

    if (!result) {
        std::cerr << "Send request failed\n";
        return false;
    }

    WinHttpReceiveResponse(hRequest, NULL);

    DWORD statusCode = 0;
    DWORD size = sizeof(statusCode);
    WinHttpQueryHeaders(hRequest,
                        WINHTTP_QUERY_STATUS_CODE | WINHTTP_QUERY_FLAG_NUMBER,
                        WINHTTP_HEADER_NAME_BY_INDEX,
                        &statusCode, &size, WINHTTP_NO_HEADER_INDEX);

    // Release resources
    WinHttpCloseHandle(hRequest);
    WinHttpCloseHandle(hConnect);
    WinHttpCloseHandle(hSession);

    return (statusCode == 200);
}

int main() {
    std::wstring webhookUrl = L"https://hooks.slack.com/services/xxx/yyy/zzz"; // Replace with your Webhook URL

    std::string message = R"({
        "text": "Hello from C++ :rocket:",
        "username": "C++ Bot",
        "icon_emoji": ":robot_face:"
    })";

    if (PostToSlack(webhookUrl, message)) {
        std::cout << "Posted successfully!\n";
    } else {
        std::cerr << "Failed to post.\n";
    }

    return 0;
}

🧪 Customizing JSON Messages

With Slack Webhooks, you can include parameters like the following:

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{
  "text": "Notification content",
  "username": "Bot name",
  "icon_emoji": ":rocket:",
  "channel": "#desired_channel_name"
}

📌 Supplementary Notes

Content-Type must be specified as "application/json"
Pass the Webhook URL as a wstring without any changes (URL encoding is unnecessary)
Since it’s HTTPS communication, don’t forget WINHTTP_FLAG_SECURE

🎉 Bonus: Example of Post Confirmation in Slack

It will be displayed in Slack like this:

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[C++ Bot]
Hello from C++ :rocket:

✍️ Summary

Item	Details
Posting Method	Webhook (Incoming Webhooks)
Communication Library	WinHTTP
Data Format	JSON
Usable Parameters	text, username, icon_emoji, channel, etc.

Even if you thought “Integrating C++ with Slack? No way…”, you can start embedding a notification bot today!

🚀 Teaser for Next Time?

If you’re interested, next time:

File attachments
UI with buttons
Flexible API operations with Slack App + OAuth2

I can introduce you to more advanced Slack integrations like these!