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56 lines
1.8 KiB
Markdown
56 lines
1.8 KiB
Markdown
---
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title: "S16. Detect When the Client Has Disconnected"
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order: 35
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status: "draft"
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---
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During a long-running response, the client might close the connection. There's no point continuing to do work no one's waiting for. In cpp-httplib, check `req.is_connection_closed()`.
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## Basic usage
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```cpp
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svr.Get("/long-task", [](const httplib::Request &req, httplib::Response &res) {
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for (int i = 0; i < 1000; ++i) {
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if (req.is_connection_closed()) {
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std::cout << "client disconnected" << std::endl;
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return;
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}
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do_heavy_work(i);
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}
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res.set_content("done", "text/plain");
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});
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```
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`is_connection_closed` is a `std::function<bool()>`, so call it with `()`. It returns `true` when the client is gone.
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## With a streaming response
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The same check works inside `set_chunked_content_provider()`. Capture the request by reference.
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```cpp
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svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
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res.set_chunked_content_provider(
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"text/event-stream",
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[&req](size_t offset, httplib::DataSink &sink) {
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if (req.is_connection_closed()) {
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sink.done();
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return true;
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}
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auto event = generate_next_event();
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sink.write(event.data(), event.size());
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return true;
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});
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});
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```
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When you detect a disconnect, call `sink.done()` to stop the provider from being called again.
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## How often should you check?
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The call itself is cheap, but calling it in a tight inner loop doesn't add much value. Check at **boundaries where interrupting is safe** — after producing a chunk, after a database query, etc.
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> **Warning:** `is_connection_closed()` is not guaranteed to reflect reality instantly. Because of how TCP works, sometimes you only notice the disconnect when you try to send. Don't expect pixel-perfect real-time detection — think of it as "we'll notice eventually."
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