Compile-time validation

Whenever possible, I try to ensure the validation of my program early. I can move some verifications from runtime to compile-time. As such, I will find some issues when I compile the program instead of while running it.

With good tests, I will still find issues earlier than the user. But with human error, some unpleasant cases can reach the user if tests do not cover them. And they might not always be easy to spot.


Take the following situation:

    • A third-party library provides an std::array with some data (integers).
    • I convert this data into another std::array that my application owns (instances of a struct).

It’s as simple as it sounds.

#include <algorithm>
#include <array>
#include <cassert>
#include <numeric>

namespace lib {
using A = std::array<int, 9>;

inline A fetch()
    A a;
    std::iota(a.begin(), a.end(), 1);
    return a;
}  // namespace lib

namespace app {
struct S {
    int i = 0;
    S() = default;
    explicit S(int v) noexcept : i{v} {}
inline bool operator==(int i, S s) noexcept { return i == s.i; }

using B = std::array<S, 9>;

inline B convert(const lib::A& a)
    B b;
    std::transform(a.cbegin(), a.cend(), b.begin(), [](int i) noexcept { return S{i}; });

    return b;
}  // namespace app

int main()
    const auto a = lib::fetch();
    const auto b = app::convert(a);

    assert(std::equal(a.cbegin(), a.cend(), b.cbegin()));


The issue that might not be too easy to spot, especially if the code is more complicated, is that the third-party library could change the array’s size.

My code would still compile. And of course, given that I have a test (represented here by the assert), it would fail. But I would not know why it failed until I debug the code. And if I don’t have a test, I’m not covered.

Specifically for my code, if the third-party library’s array would get larger than my app’s array, I would get an undefined behavior because I would try to copy data beyond my array’s bounds.

Moreover, a change like that in the third-party library might be something really important for me and it should scream in my face. Continue reading Compile-time validation