C++ New Added 9 October 2026

C++: terminate called after throwing an instance of 'std::bad_alloc'

An allocation asked for more memory than the allocator could provide, nothing caught the exception, and the runtime aborted. On Linux, real memory exhaustion usually brings the OOM killer instead, so this message more often means one absurd request: a negative length converted to size_t, a size read from uninitialised memory, or a 32-bit build out of address space. A ulimit -v on the process produces it for genuine shortage.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Stop at the throw and read the requested size from the backtrace
gdb ./app -ex 'catch throw' -ex run -ex bt

# AddressSanitizer reports oversized requests with size and call site
g++ -g -fsanitize=address app.cpp -o app && ./app
# look for "requested allocation size ... exceeds maximum supported size"

# Was the process given a memory limit?
ulimit -v

// Typical bug: a failed read returns -1, which becomes ~18 exabytes
if (len < 0) throw std::runtime_error("read failed");
std::vector<char> buf(static_cast<std::size_t>(len));

How to diagnose C++ errors

C++ errors fall into two very different worlds. Compile and link errors (undefined reference, multiple definition, template substitution failure) are deterministic and are almost always about declarations, the One Definition Rule, or link order. Runtime memory errors (segfaults, double free, use-after-free) are non-deterministic and should never be debugged by reading code alone; a sanitizer will find in seconds what code review misses for days.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Rebuild with sanitizers before anything else: -fsanitize=address,undefined -fno-omit-frame-pointer -g. AddressSanitizer reports the exact allocation and free sites for use-after-free and double-free.
For undefined-reference errors, check link order, remembering that with GNU ld libraries must come after the objects that use them, and check for a C/C++ linkage mismatch that needs extern "C".
Use nm -C libfoo.a | grep symbol to confirm the symbol is actually present and to see the demangled signature. A signature that differs by a const is a different symbol.
Enable core dumps (ulimit -c unlimited) and open them in gdb with gdb ./binary core, then bt full. A stack trace beats speculation.
For template errors, read the message from the bottom up. The final line is usually the real constraint that failed; everything above is instantiation context.

Tools worth reaching for

-fsanitize=address,undefined
valgrind
gdb / lldb
nm -C
ldd

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

en.cppreference.com clang.llvm.org

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