Can Local Classes Finally Be Used as Predicates for STL Algorithms in C 11?

Using Local Classes with STL Algorithms: Unraveling the Local Type Restriction

In the realm of C programming, local classes have long been disallowed as predicates for STL algorithms, raising questions about the underlying restriction and its rationale.

The C 98/03 standard explicitly prohibits local types as template arguments, including local classes. Article 14.3.1 of the standard unequivocally states that "local types [...] shall not be used as a template-argument for a template type-parameter."

The rationale behind this restriction stems from the ephemeral nature of local types. As they exist within a limited scope, their destructions can occur unexpectedly, potentially leading to undefined behavior when used as predicates for STL algorithms.

However, with the advent of C 11, this restriction was lifted. The revised standard allows for local types to be used as template arguments, enabling local classes to serve as predicates for STL algorithms.

For example, consider the following code snippet:

#include <algorithm>
#include <vector>

int main() {
    int array[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    std::vector<int> v(array, array + 10);

    struct even {
        bool operator()(int x) { return !(x % 2); }
    };
    std::remove_if(v.begin(), v.end(), even());
}

In this code, the local class even defines a predicate that checks for even numbers. This class can now be used directly as an argument to the std::remove_if algorithm, which removes all even numbers from the vector v.

While the restriction on local types was initially intended to safeguard against potential undefined behavior, its removal in C 11 has opened up new possibilities for using local classes in functional programming scenarios.

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