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I want my function to take a lvalue reference and absolutely not a rvalue or temporary or whatever.

This is my function:

template<class T> void foo(T& value) {} // Imagine I have a class Foo struct Foo { int a; int b; };

When I call foo(Foo{1, 2}), first, it compiles even if I asked for a lvalue reference, and second, it doesn't work because foo stores the address of the passed value, so I get garbage when I read it later.

How to force foo to take a lvalue reference?

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7
  • Look at stackoverflow.com/questions/50852470/… Commented Jun 24, 2020 at 20:28
  • 4
    With your given code snippets foo(Foo(1, 2)) does not compile. Commented Jun 24, 2020 at 20:29
  • 1
    learn.microsoft.com/en-us/cpp/build/reference/… Commented Jun 24, 2020 at 20:29
  • What compiler are you using? VS2019 emits a warning C4239: nonstandard extension used: 'argument': conversion from 'Foo' to 'T &' or an error C2664: 'void foo<Foo>(T &)': cannot convert argument 1 from 'Foo' to 'T &', depending on if languages extensions are enabled or disabled. Commented Jun 24, 2020 at 20:31
  • That doesn't work at all: See on coliru Commented Jun 24, 2020 at 20:31

1 Answer 1

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6

First of all, the given code will not compile by default in GCC and Clang, only in MSVC with non-standard extensions.

If you want to make sure that foo() does not accept an rvalue, just delete that overload:

template<class T> void foo(T&& value) = delete;
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1 Comment

That’s a universal reference, so it triggers regardless of value category. You’re relying on partial ordering to prefer the lvalue-reference choice; better would be to constrain the deleted function template so that it doesn’t outcompete normal functions that require conversions.

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