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I was under the impression that the following should become valid code under the new C++20 standard:
struct Foo
{
int a, b;
};
template<Foo>
struct Bar
{};
Bar<{.a=1, .b=2}> bar;
Yet, gcc 10.2.0, with -std=c++20 set complains: could not convert ‘{1, 2}’ from ‘<brace-enclosed initializer list>’ to ‘Foo’ and Clang cannot compile this snippet either. Can someone point out why it is not well formed?
377
A template non-type parameter is a template parameter where the type of the parameter is predefined and is substituted for a constexpr value passed in as an argument. A non-type parameter can be any of the following types: An integral type. An enumeration type. A pointer or reference to a class object.
In C++ this can be achieved using template parameters. A template parameter is a special kind of parameter that can be used to pass a type as argument: just like regular function parameters can be used to pass values to a function, template parameters allow to pass also types to a function.
For example, given a specialization Stack<int>, “int” is a template argument. Instantiation: This is when the compiler generates a regular class, method, or function by substituting each of the template's parameters with a concrete type.
This template-argument
{.a=1, .b=2}
is not allowed according to the grammar for a template-argument which only allows the following constructs:
template-argument:
constant-expression
type-id
id-expression
A brace-init list is not any of the above constructs, it's actually an initializer and so it cannot be used as a template-argument.
You can be explicit about the type of the object that you use as the template-argument:
Bar<Foo{.a=1, .b=2}> bar;
and this will work, since this is a constant-expression.
190
It's a C++ grammar thing. Things used for template arguments must be either type-ids, id-names, or constant-expressions.
And a braced-init-list is not an expression of any kind. They can only grammatically be present in a small number of places, specifically those used to initialize an object or variable.
In the past, it wasn't really relevant, since there wasn't much reason to use a braced-init-list to initialize the few valid NTTPs. Obviously that's changed, so this is something of an oversight. But this is what the C++20 standard says.
37
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