Cannot std::move from lambda capture to function call inside lambda, why?

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In the following code

#include <memory> #include <thread> #include <utility>  void f1(std::unique_ptr<int>&& uptr) {}  void f(std::unique_ptr<int>&& uptr) {     auto thread = std::thread([uptr{ std::move(uptr) }]() {         f1(std::move(uptr));     }); }  int main() {     return 0; } 

the call to std::move inside the lambda cannot be compiled:

[x86-64 gcc 8.1 #1] error: binding reference of type'std::unique_ptr<int>&&' to 'std::remove_reference<const> std::unique_ptr<int>&>::type' {aka 'const std::unique_ptr<int>'} discards qualifiers 

Live demo:

Why does this error occur and how can I fix it? Where does const come from?


You need to make the lamdba mutable as the closure variables are const-qualified by default.

auto thread = std::thread([uptr{ std::move(uptr) }]() mutable {                                                     //^^^^^^       f1(std::move(uptr)); /* Works, no constness-violation.*/  }); 


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