what are the differences between returning auto, auto&, auto&& and decltype(auto)?

JUANDENT (404)
what are the differences between returning auto, auto&, auto&& and decltype(auto)?

for instance:

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auto f(int val) { 
static int value = val;
return value;
}
auto& f(int val) { 
static int value = val;
return value;
}
auto&& f(int val) { 
static int value = val;
return value;
}
decltype(auto) f(int val) { 
static int value = val;
return value;
}


??
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