C++: error: use of deleted function 'X::X(const X&)' — copying a non-copyable type

Meaning

A type becomes non-copyable when a member is non-copyable, when it has a user-declared move operation, or when = delete is written. Passing it by value, returning a local from a lambda, or storing it in a container that copies triggers the error. Move it, pass by reference or pointer, or make the type copyable.

Common causes

• Copying a std::unique_ptr, std::mutex, std::thread or std::ifstream
• Class with a non-copyable member is copied implicitly
• Constructor or assignment explicitly = delete
• Passing the object by value to a function or lambda capture by copy

Detailed fix by platform

Code examples

#include <memory>

void take(std::unique_ptr<int> p);

int main() {
    auto a = std::make_unique<int>(42);
    // take(a);                 // error: use of deleted function 'unique_ptr(const unique_ptr&)'
    take(std::move(a));         // OK: ownership moves, 'a' is now empty
}

How to diagnose

1. Type — Which type's copy is deleted?
2. Call site — Where is a copy made implicitly?
3. Intent — Should it be moved, shared or referenced?

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