49 lines
3.2 KiB
Markdown
49 lines
3.2 KiB
Markdown
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# ADR-002: Interface-Based Dependency Injection Over Preprocessor Mocking
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## Status: Accepted
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## Context
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In C++ ecosystems, mocking frameworks like Google Mock (gmock) use template metaprogramming, macros, and preprocessor directives to generate mock classes at compile time. This approach is unavailable in MQL5 because:
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1. MQL5 has no template metaprogramming. While the language supports basic template<typename T> syntax for generic functions and classes, it lacks template specialization, SFINAE (Substitution Failure Is Not An Error), variadic templates, constexpr evaluation, and type traits — all essential for compile-time mock generation.
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2. MQL5 has no reflection or runtime type introspection. There is no typeid, no RTTI, and no way to enumerate or inspect the methods of a class at runtime, making automatic mock class generation impossible.
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3. MQL5's preprocessor is too limited for mock generation. Although it supports #define (including function-like macros with arguments), #if, #else, #elif, #ifdef, #ifndef, #undef, #include, and #pragma, it lacks token pasting (##), stringification (#), and variadic macros (__VA_ARGS__) — all of which are required to programmatically generate mock class definitions and method stubs.
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We need a mechanism to test Expert Advisor logic **without a live MetaTrader 5 terminal**, meaning all dependencies on native global functions (`SymbolInfoDouble()`, `TimeCurrent()`, `OrderSend()`) must be replaceable.
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## Decision
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Use **pure virtual interfaces** (MQL5 `interface` keyword) with **constructor injection**:
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```mql5
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interface IMarketEnvironment
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{
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double GetBid(void);
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double GetAsk(void);
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bool TryExecuteOrder(MqlTradeRequest &request, MqlTradeResult &result);
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};
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// Test implementation
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class CSyntheticEngine : public IMarketEnvironment { ... };
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// Future production implementation
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class CLiveEnvironment : public IMarketEnvironment { ... };
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```
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The Expert Advisor receives an `IMarketEnvironment*` in its constructor (or initialization method), never calling native functions directly. Swapping between test and production is a single line change at the composition root.
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## Consequences
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### Positive
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- **Complete isolation**: EA logic can be tested with zero terminal dependencies.
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- **Open/Closed Principle**: new tick sources (`CFileProvider`, `CRandomProvider`) extend `ITickProvider` without modifying the engine.
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- **Explicit dependencies**: every dependency is visible in the constructor signature — no hidden globals.
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- **Familiar pattern**: developers who know Strategy Pattern, Ports & Adapters, or Hexagonal Architecture will recognize this immediately.
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### Negative
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- **Vtable overhead**: virtual method dispatch adds ~2-5 nanoseconds per call compared to direct function calls. For test scenarios (microsecond-scale), this is negligible. For high-frequency live trading, profiling is recommended.
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- **Boilerplate**: each native function abstracted requires a method on the interface + implementations in both test and live adapters. This is the "tax" of testability.
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- **Single inheritance limitation**: MQL5 allows single class inheritance + multiple interface implementation. Complex hierarchies may hit this wall, but the current design stays flat.
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