Setting errno in `operator T`

Paul McKenney and JF Bastien write in P0108 “Skeleton Proposal for Thread-Local Storage (TLS)” (April 2016):

The preferred approach is to provide alternative wrappers for the functions […] Code sensitive to errno could assign the return value to a status_value and then extract both the errno and the return value.

They propose essentially this code:

template <typename T>
class math_result {
    math_error e;
    T val;

public:
    explicit operator math_error() const {
        return e;
    }
    operator T() const {
        return val;
    }
};

extern double tgamma(double, math_error *);

math_result<double> tgamma(double x) {
    math_result<double> ret;
    ret.val = tgamma(x, &ret.e);
    return ret;
}

int main() {
    double x = tgamma(3.14);  // discards errno
    auto xe = tgamma(3.14);
    printf("%d\n", int(math_error(xe)));
}

But you know what might be even better IMHO? This code:

template <typename T>
class math_result {
    math_error e;
    T val;

public:
    math_error error() const { return e; }
    T value() const { return val; }
    operator T() const {
        if (e) errno = e;
        return val;
    }
};

extern double tgamma(double, math_error *);

math_result<double> tgamma(double x) {
    math_result<double> ret;
    ret.val = tgamma(x, &ret.e);
    return ret;
}

int main() {
    double x = tgamma(3.14);  // sets errno in a POSIX-compatible manner
    double y = tgamma(3.14).value();  // discards errno
    auto xe = tgamma(3.14);
    printf("%d\n", int(xe.error()));
}

This math_result::operator T sets errno if you try to retrieve the value implicitly from a result, similarly to how std::expected::value() throws an exception if you try to retrieve the value from a valueless expected. The theme is the same: take nice value-semantic local behavior and, for historical reasons, when misused, turn it into icky global-side-effecting behavior.

Posted 2018-06-01

thread-local-storage

Setting errno in operator T

Setting errno in `operator T`