C++ New Added 29 September 2026

C++: error: use of deleted function

The code tries to call a function that has been explicitly deleted, and nine times out of ten that function is a copy constructor. Types such as std::unique_ptr, std::thread and std::mutex cannot be copied, so passing one by value, pushing it into a vector or capturing it in a lambda by copy triggers this. A class that holds one of them has its own copy constructor deleted too, which is why the error can point at code that never mentions the member.

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

// Read the 'note:' lines under the error: they name the member that
// made the copy constructor deleted, e.g. std::unique_ptr<Conn> conn_;

// Move instead of copying
std::unique_ptr<Conn> c = std::make_unique<Conn>();
conns.push_back(std::move(c));

// Take by reference, or by value and move in
void use(const Conn& c);
void take(std::unique_ptr<Conn> c);
take(std::move(c));

// Lambdas: move-capture
auto job = [c = std::move(c)] { c->send(); };

# Get the full chain of notes from the compiler
g++ -std=c++20 -fdiagnostics-show-template-tree -c main.cpp

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 en.cppreference.com

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