3. Dependency injection¶

Baldr uses Skirnir for dependency injection. Services registered with the application builder are available to handlers via their parameter list — no manual lookup.

Register a service¶

src/main.cpp
#include <Baldr/Baldr.hpp>

class Clock
{
  public:
    virtual ~Clock() = default;
    virtual std::string now() const = 0;
};

class SystemClock final : public Clock
{
  public:
    std::string now() const override
    {
        return "2026-01-01T00:00:00Z";
    }
};

int main()
{
    auto builder =
        skr::ApplicationBuilder().WithExtension<baldr::BaldrExtension>();

    builder.GetServiceCollection()->AddSingleton<Clock>(
        skr::MakeArc<SystemClock>());

    auto app = builder.Build<baldr::WebApplication>();

    app->MapGet("/now", [](skr::Arc<Clock> clock) {
        return std::string(clock->now());
    });

    app->Run();
    return 0;
}

The handler takes skr::Arc<Clock> as a parameter and Skirnir resolves the registered SystemClock instance per request. No globals, no manual new.

Service lifetimes¶

Method	Behaviour
`AddSingleton<T>(...)`	One instance shared by every request.
`AddScoped<T>(...)`	One instance per request scope.
`AddTransient<T>(...)`	New instance every resolution.

For most applications AddSingleton is the right choice — handlers are stateless and benefit from sharing immutable collaborators.

Multiple implementations¶

Register one type and resolve another:

src/main.cpp

class IUserRepository { /* ... */ };
class InMemoryUserRepository final : public IUserRepository { /* ... */ };

// Register the interface; resolve by interface in handlers.
builder.GetServiceCollection()->AddSingleton<IUserRepository>(
    skr::MakeArc<InMemoryUserRepository>());

Swap InMemoryUserRepository for SqlUserRepository in one place without touching handlers.

Next¶

Continue with 4. Middleware.