Can I Use Local Classes as Predicates in STL Algorithms?

Local Classes in STL Algorithms

In the realm of C programming, the Standard Template Library (STL) offers a comprehensive set of algorithms for manipulating data structures. However, a common question arises regarding the use of locally defined classes as predicates within STL algorithms.

The Prohibition in Pre-C 11 Standards

Historically, the C 98/03 standards explicitly prohibited the use of local types as template arguments for STL algorithms. This restriction extended to local classes. For example:

struct even : public std::unary_function<int,bool> {
  bool operator()(int x) { return !(x % 2); }
};
std::remove_if(v.begin(), v.end(), even()); // error

According to the standard, "A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template-argument for a template type-parameter."

The Rationale Behind the Restriction

The original rationale for this restriction is unclear. It may have been due to potential ambiguity or technical limitations with the implementation of template metaprogramming in early versions of the language.

The Relaxation in C 11

Fortunately, the C 11 standard removed this restriction, allowing developers to use local classes as template arguments. This change greatly enhanced the expressiveness and flexibility of STL algorithms.

The Solution in Practice

In practice, most modern compilers now support the use of local classes with STL algorithms, even if the underlying language standard does not explicitly allow it. However, it is important to check the documentation for the specific compiler in use to ensure compatibility.

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