Quick Q: const-reference qualified member function
Quick A: Delete the r-value reference overload of the function.
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const-reference qualified member function
A temporary can be bound to a
const&qualified object and the ref-qualifier effectively qualifies the implicitly passed object (*this). If you want to prevent calls on temporaries but allow lvalues, you can= deletethe rvalue reference overload and implement the lvalue version. Using const qualified reference qualifiers for both operators requires just one implemented and one= deletedimplementation:class File { // ... FILE* _file; public: operator FILE*() const&& = delete; operator FILE*() const& { return this->_file; } // ... };The net-effect is that you can use the conversion only for objects to which you go an lvalue:
int main() { File f; File const cf{}; FILE* fp = f; // OK FILE* cfp = cf; // OK FILE* tfp = File(); // ERROR: conversion is deleted FILE* mfp = std::move(cf); // ERROR: conversion is deleted }
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