Why Does Argument Dependent Lookup Fail to Find Function Templates with Explicit Template Arguments in C ?

Argument Dependent Lookup and Function Templates in C

Argument dependent lookup (ADL) is a language feature that enables the compiler to search for identifiers within the namespaces associated with the arguments of a function call. While ADL typically finds functions, it fails to find function templates in some scenarios.

The C Standard Explanation

Section 14.8.1.6 of the C Standard (2003) states that:

"But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply."

Example

Consider the following example:

namespace ns {
    struct foo {};
    template<int i> void frob(foo const&amp;) {}
    void non_template(foo const&amp;) {}
}

int main() {
    ns::foo f;
    non_template(f); // This is fine.
    frob<0>(f); // This is not.
}

The last call in main (frob<0>(f);) fails to compile because no function template with the name frob is visible in the scope of the call. ADL cannot find frob<0> because function templates are not considered valid function calls by the compiler's syntax.

Additional Considerations

• For simple function names (without template arguments), ADL can still find function templates even if their names are not visible.
• However, if the function template is called with explicit template arguments, a function template with that specific name must be visible in the scope of the call for ADL to succeed.
• In some cases, using declarations can make a function template visible, allowing ADL to find it.

The above is the detailed content of Why Does Argument Dependent Lookup Fail to Find Function Templates with Explicit Template Arguments in C ?. For more information, please follow other related articles on the PHP Chinese website!