cpp-di

A minimalistic C++17 dependency injection framework.

Motivation

A key characteristic of software is that it needs to be changed over time as requirements evolve. By de-coupling different parts of a program from each other, we can reduce the effort that comes with such changes. The resuling architecture often looks like this:

• The program is divided into components.
• Component interface and implementation are separate.
• Dependencies between components are against interfaces, not implementations.

In this context, dependency injection solves the problem of constructing components and assembling them in a flexible and convenient way.

Usage

1. Setup

Add di.h and di.cpp to your project and

#include "di.h"

2. Create interfaces

class Greeter
{
  virtual void greet() = 0;
};

class Printer
{
  virtual void print(std::string_view text) = 0;
};

3. Create implementations

class GreeterImpl : public Greeter
{
  void greet() override
  {
    printer->print("Hello!");
  }
  
  di::ServiceRef<Printer> printer;
};

class ConsolePrinterImpl : public Printer
{
  void print(std::string_view text) override
  {
    std::cout << text << std::endl;
  }
};

class FilePrinterImpl : public Printer
{
  FilePrinterImpl(std::string_view fn) { f.open(std::string(fn)); }
  ~FilePrinterImpl() { f.close(); }
  
  void print(std::string_view text) override
  {
    f << text << std::endl;
  }
  
  std::ofstream f;
};

4. Define bindings

auto consoleApp = di::Bindings{}
  .service<Greeter, GreeterImpl>()
  .service<Printer, ConsolePrinterImpl>();

auto loggingApp = di::Bindings{}
  .service<Greeter, GreeterImpl>()
  .service<Printer, FilePrinterImpl>("log.txt");

5. Create instances within a scope

{
  auto scope = di::Scope{consoleApp};
  di::ServiceRef<Greeter> greeter;
  greeter->greet(); // Greets to console
}

{
  auto scope = di::Scope{loggingApp};
  di::ServiceRef<Greeter> greeter;
  greeter->greet(); // Greets to log.txt
}

Status

This framework is work-in-progress and should not be used in production yet.