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68 lines
2.4 KiB
Markdown
68 lines
2.4 KiB
Markdown
---
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title: "S14. Catch Exceptions"
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order: 33
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status: "draft"
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---
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When a route handler throws, cpp-httplib keeps the server running and responds with 500. By default, though, very little of the error information reaches the client. `set_exception_handler()` lets you intercept exceptions and build your own response.
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## Basic usage
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```cpp
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svr.set_exception_handler(
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[](const httplib::Request &req, httplib::Response &res,
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std::exception_ptr ep) {
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try {
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std::rethrow_exception(ep);
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} catch (const std::exception &e) {
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res.status = 500;
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res.set_content(std::string("error: ") + e.what(), "text/plain");
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} catch (...) {
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res.status = 500;
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res.set_content("unknown error", "text/plain");
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}
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});
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```
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The handler receives a `std::exception_ptr`. The idiomatic move is to rethrow it with `std::rethrow_exception()` and catch by type. You can vary status code and message based on the exception type.
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## Branch on custom exception types
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If you throw your own exception types, you can map them to 400 or 404 responses.
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```cpp
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struct NotFound : std::runtime_error {
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using std::runtime_error::runtime_error;
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};
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struct BadRequest : std::runtime_error {
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using std::runtime_error::runtime_error;
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};
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svr.set_exception_handler(
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[](const auto &req, auto &res, std::exception_ptr ep) {
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try {
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std::rethrow_exception(ep);
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} catch (const NotFound &e) {
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res.status = 404;
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res.set_content(e.what(), "text/plain");
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} catch (const BadRequest &e) {
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res.status = 400;
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res.set_content(e.what(), "text/plain");
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} catch (const std::exception &e) {
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res.status = 500;
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res.set_content("internal error", "text/plain");
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}
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});
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```
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Now throwing `NotFound("user not found")` inside a handler is enough to return 404. No per-handler try/catch needed.
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## Relationship with set_error_handler
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`set_exception_handler()` runs the moment the exception is thrown. After that, if `res.status` is 4xx or 5xx, `set_error_handler()` also runs. The order is `exception_handler` → `error_handler`. Think of their roles as:
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- **Exception handler**: interpret the exception, set the status and message
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- **Error handler**: see the status and wrap it in the shared template
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> **Note:** Without an exception handler, cpp-httplib returns a default 500 response and the exception details never make it to logs. Always set one for anything you want to debug.
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