Document number: P2370R0
Project: Programming Language C++
Audience: LEWG Incubator, LEWG, LWG
Andrei Nekrashevich <>, <>
Antony Polukhin <>, <>
Date: 2021-05-25

Stacktrace from exception

I. Introduction and Motivation

Quite often Standard Library exceptions do not provide enough information to diagnose the error. Consider the example:

#include <iostream>
#include <stdexcept>
#include <string_view>

void foo(std::string_view key);
void bar(std::string_view key);

int main() {
  try {
  } catch (const std::exception& exc) {
    std::cerr << "Caught exception: " << exc.what() << '\n';

The output of the above sample may be the following:

Caught exception: map::at

That output is quite useless because it does not help the developer to understand in what function the error happened.

This paper proposes to add an ability to get stacktrace of a caught exception, namely to add static method std::stacktrace::from_current_exception():

#include <iostream>
#include <stdexcept>
#include <string_view>
#include <stacktrace>   // <---

void foo(std::string_view key);
void bar(std::string_view key);

int main() {
  try {
  } catch (const std::exception& exc) {
    std::stacktrace trace = std::stacktrace::from_current_exception();  // <---
    std::cerr << "Caught exception: " << exc.what() << ", trace:\n" << trace;

The output of the above sample may be the following:

Caught exception: map::at, trace:
 0# get_data_from_config(std::string_view) at /home/axolm/basic.cpp:600
 1# bar(std::string_view) at /home/axolm/basic.cpp:6
 2# main at /home/axolm/basic.cpp:17

That output is quite useful! Without a debugger we can locate the source file and the function that has thrown the exception.

More production log examples where stacktraces would help to diagnose the problem faster:

Here's another motivation example. It allows the developer to diagnose std::terminate calls because of throws in noexcept function:

#include <iostream>
#include <stacktrace>
#include <stdexcept>
#include <unordered_map>

void broken_function() noexcept {
  std::unordered_map<std::string, int> m;
  [[maybe_unused]] auto value ="non-existing-key");

int main() {
  std::set_terminate([] {
    auto trace = std::stacktrace::from_current_exception();
    if (trace) {
        std::cerr << "Terminate was called with an active exception:\n"
                  << trace << std::endl;



Exception trace:
 0# std::__throw_out_of_range(char const*) at /build/gcc/src/gcc/libstdc++-v3/src/c++11/
 1# std::__detail::_Map_base<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator ...
 2# std::unordered_map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > ...
 3# broken_function() at /home/axolm/terminate.cpp:8
 4# main at /home/axolm/terminate.cpp:17

Disclaimer: may not produce traces on all platforms

Finally, an ability to extract stacktraces from exceptions addresses feature requests from "2021 Annual C++ Developer Survey". In it some people were complaining that in their projects exceptions are restricted due to lack of stacktraces.

II. Implementation and ABI stability

Majority of the popular platforms have special functions to allocate/throw/deallocate exceptions: __cxa_allocate_exception, __cxa_throw, __CxxThrowException... To embed a trace into an exception an underlying implementation may be changed, to gather the trace at the point where the exception is thrown.

This can be done without breaking the ABI. Here's a prototype libsfe that replaces those functions and adds a stacktrace into each exception. Application can switch between exceptions with traces and without via adding and removing a LD_PRELOAD=/usr/local/lib/

III. Use cases

The table bellow shows std::stacktrace related techniques to use in different cases

Case HowTo Proposal
Need a way to get diagnostic info for an unrecoverable errors. For example for failed assert checks Just log the trace and abort:
std::cerr << std::stacktrace::current();
Need to get stacktraces from some code paths and you have control over the throw site. Manually embed the stacktrace into the exception:
template <class Base>
struct TracedException: Base, std::stacktrace {
  TracedException(): Base(), std::stacktrace(std::stacktrace::current()) {}
// ...
throw TracedException<std::bad_any_cast>();
Extract it at the catch site:
catch (const std::exception& e) {
    if (auto ptr = dynamic_cast<const std::stacktrace*>(&e); ptr) {
        std::cerr << *ptr;
    // ...
Need a trace from the throw site to simplify diagnoses of failed tests. Use stacktrace::from_current_exception in your testing macro to get the trace:
This proposal
Need a way to diagnose a running application in production. Turn on stacktrace::from_current_exception in your error handling logic:
try {
} catch (const std::exception& e) {
    if (auto trace = std::stacktrace::from_current_exception(); trace)
        LOG_PROD_DEBUG() << e.what() << " at " << trace;
This proposal

To sum up: you need functionality from this paper when you have no control over the throw site or changing all the throw sites is time consuming.

IV. Design decisions

A. Recommend to turn off by default

As was noted in mailing list discussion the proposed functionality increases memory consumption by exceptions. Moreover, it adds noticeable overhead on some platforms.

Because of that we recommend to disable the functionality by default in release builds and enable in debug. To enable the stacktrace capturing on exception object construction for the current thread user would have to use std::this_thread::capture_stacktraces_at_throw(bool enable) noexcept;.

B. Safety

Proposed std::stacktrace::from_current_exception() function is a diagnostic facility. It would be used in error processing and other places that are rarely tested. Because of that it follows the std::stacktrace design: do not report missing traces as errors, do not throw and treat all the internal errors as a missing trace.

C. Link time flag from std::stacktrace

"A Proposal to add stacktrace library" P0881 encouraged implementations to provide a link time flag to disable or enable traces. With disabled tracing std::stacktrace::from_current_exception() may be called at runtime and it returns default constructed std::stacktrace.

D. Per thread ability to enable

Consider some server under heavy load with 5 different executors. Something goes wrong with one of the executors. Capturing stacktraces on exception object construction and symbolizing them for all the executors could slow down the server significantly while only traces for 1 executor are needed. So the option should be thread specific.

Such approach scales well to coroutines. Call the std::this_thread::capture_stacktraces_at_throw(bool enable) noexcept; after context switch and you have per coroutine ability to enable or disable stacktrace capturing at the exception object construction.

V. Wording

Add to the [stacktrace.syn]:

  // [stacktrace.basic], class template basic_stacktrace
  template<class Allocator>
    class basic_stacktrace;

  namespace this_thread {
    void capture_stacktraces_at_throw(bool enable = true) noexcept;

  // basic_stacktrace typedef names
  using stacktrace = basic_stacktrace<allocator<stacktrace_entry>;>;

Add to the [stacktrace.basic.overview]:

    static basic_stacktrace current(size_type skip, size_type max_depth,
                                    const allocator_type& alloc = allocator_type()) noexcept;

    static basic_stacktrace from_current_exception(
                                    const allocator_type& alloc = allocator_type()) noexcept;

    basic_stacktrace() noexcept(is_nothrow_default_constructible_v<allocator_type>);

Add to the [stacktrace.basic.ctor] after the description of current() functions:

static basic_stacktrace from_current_exception(const allocator_type& alloc = allocator_type()) noexcept;
Effects: Returns a basic_stacktrace object with frames_ storing the stacktrace of the evaluation at the point of construction of the currently handled exception, or an empty basic_stacktrace object if the initialization of frames_ failed, stacktrace captures were not enabled for the throwing thread or no exception is being handled. alloc is passed to the constructor of the frames_ object.
[Note: Rethrowing exceptions with throw; do not change stacktrace, throwing exception with throw e; stores a new stacktrace.- end note]

Add section [stacktrace.thread.this] before the [stacktrace.basic.mod] section:

Namespace this_thread [stacktrace.thread.this]

void this_thread::capture_stacktraces_at_throw(bool enable = true) noexcept;
Effects: Invoking the function with the enable parameter equal to true enables capturing stacktraces by the current thread of execution at exception object construction, disables otherwise.
Recommended practice: Implementations should disable the capturing stacktraces by the current thread of execution at exception object construction in release builds by default and enable in debug builds.

VI. Revision History

Revision 0:

VII. References

[libsfe] Proof of concept implementation. Available online at