GeneralPurpose-MutablePredicate

Non-static data members or captured values of predicate function objects that are state related to this object’s identity shall not be copied

Required inputs: IR

Function objects (functors) and lambda expressions used as predicates in algorithms must not modify state. Modifying state in a predicate violates the expectation that predicates are pure functions with no side effects. When predicates have mutable state, the behavior of algorithms becomes unpredictable and results may depend on implementation details like evaluation order.
Bad code (predicate with mutable state):
struct CountingPredicate {
    mutable int count = 0;
    bool operator()(int x) const {
        ++count;  // ERROR: modifies mutable member
        if (count % 2) {
            return x > 0;
        } else {
            return x < 0;
        }
    }
};

std::vector<int> v{-1, 0, 1, 2};
std::find_if(v.begin(), v.end(), CountingPredicate());
// Result depends on when and how many times predicate is called
Good code (stateless predicate):
struct SimplePredicate {
    bool operator()(int x) const {
        return x > 0;  // OK: pure function, no state modification
    }
};

std::vector<int> v{-1, 0, 1, 2};
auto it = std::find_if(v.begin(), v.end(), SimplePredicate());
// Deterministic and predictable
Good code (using lambda without state):
std::vector v{-1, 0, 1, 2};
auto it = std::find_if(v.begin(), v.end(),
    [](int x) { return x > 0; });  // OK: stateless lambda

Possible Messages

Key

Text

Severity

Disabled

modifies_state

Call operator of predicate passed to {} modifies state.

None

False

nonconst_op

Call operator of predicate passed to {} is not const.

None

False

Options