Tuesday Coding Tip 27 — Aliasing and ‘using’ in C++

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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Every hello world tutorial starts with the command using namespace std;. This command is dangerous as it allows you to bring all standard symbols into your namespace where they can collide with your own symbols. Depending on the kind of collision, the compiler might not tell you, nor can you be sure which version will be used.

#include <stdexcept>

int min(int a, int b)
{
    std::terminate();
}

#include <cmath>

int main() {
    using namespace std; // dangerous
    // better to import specific names like:
    using std::max;

    return min(10, 20); // which min will be called?
    // ::min would disambiguate to the local function
    // std::min would disambiguate to standard
}

It is better to use the using keyword to selectively bring symbols into the scope or even alias them to be sure they’re unique (note that namespaces alias differently).

#include <filesystem>

// Aliasing namespace works differently
namespace fs = std::filesystem;

// from aliasing classes
using Path = fs::path;

bool fileExists(const Path& path)
{
    return fs::exists(path);
}

The keyword can also be used to expose class template parameters for common usage.

template<class T>
class Foo {
public:
    // Exposing template type saves time
    // to those who need to get to it
    using DataType = T;

    T getItem() const { return T{}; }
};

int main() {
    Foo<int> foo;
    decltype(foo)::DataType item;
    item = foo.getItem();
}

And since C++20, you can bring enum values (selectively or in group) for better ergonomics in enum-based branching.

enum class EmployeeType {
    JuniorDev,
    MediorDev,
    SeniorDev,
    ScrumMaster,
    ProductManager
};

std::string to_string(EmployeeType et) {
    switch (et)
    {
        using enum EmployeeType; // Since C++20
        case JuniorDev: return "junior";
        case MediorDev: return "medior";
        // ...
    }
}

std::string to_string(EmployeeType et) {
    using EmployeeType::JuniorDev;

    if (et == JuniorDev) return "junior";
    // ...
}