Tuesday Coding Tip 20 — Be `explicit`!

Question

Tuesday coding tips are super short posts about various tidbits mainly from C++, but also from other programming languages I use. You can also follow the #TuesdayCodingTips hashtag on Mastodon and Linkedin.

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If you choose to add a conversion operator for your class in C++, be sure to mark it as explicit or you can get nasty side-effects that will be hard to debug. For example, having an implicit conversion operator to bool will make the operator less-than “work” for your class, meaning that class can be used as a key for std::map and suddenly, all hell breaks loose.

#include <iostream>
#include <vector>

class A {
public:
    operator bool() {
        return true;
    }
};

class B {
public:
    explicit operator bool() {
        return true;
    }
};

int main() {
    // This is ok
    std::cout << (A() < A()) << std::endl;
    A a;
    if (a); // implicit conversion allowed

    // Error: error: invalid operands to binary expression
    std::cout << (B() < B()) << std::endl;
    B b;
    if (bool(b)); // explicit conversion required

    return 0;
}

Similarly, it is good practice to mark single parameter constructors explicit to prevent implicit conversions from trivial data types or to possibly track places where your object is copied.

class A {
public:
    A(int) {}
    A(const A&) {}
};

class B {
public:
    explicit B(int) {}
    explicit B(const B&) {}
};

int main() {
    A a1 = 10; // ok
    A a2 = { 10 }; // ok
    A a3 = a1; // ok

    B b1 = 10; // error: no viable conversion, explicit constructor is not a candidate
    B b2 = { 10 }; // error
    B b3 = B{10}; // ok
    B b4 = b1; // error
    B b5 = B{b1}; // ok

    return 0;
}