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CHAPTER
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sing the step-by-step approach explained in 7, you can inject managed code parts into a C++ project. Due to the source code compatibility, you can easily call from native code to managed code by just declaring the functions you want to call. However, in the samples discussed so far, the managed functions called from native code and the native functions called from managed code have had void as a return type, and no arguments. For nontrivial scenarios, functions with less simplistic signatures are obviously needed. The first important thing you have to understand is that if you compile to native code, your source code cannot use managed types at all. Just as C doesn t know anything about C++ classes, C++ doesn t know about managed types. In the other direction, the situation looks better. Source code compiled to managed code can use all native types as well as all managed types, just as C++ code can use C structs, enums, and unions, as well as C++ classes. To say this in other words, the C++ type system is the lowest common denominator between sources compiled to native code and sources compiled to managed code. This means that you have to restrict yourself to the C++ type system if you define methods that act as migration boundaries all managed functions that should be callable from native code and all native functions that should be callable from managed code can use only native types. However, any kind of native type can be used in the method signature. You can use native classes, structs, unions, enums, pointers of any level of indirection, references, and const variants of all native types. To call a native function or use a native class, you simply include the native header with the function or the class declaration. To define a managed function that native code can call, just make sure that you use only types that C++ understands. This chapter first covers how C++/CLI maps native C++ types to managed types, which explains why this type compatibility is possible and what happens under the hood. After that, it discusses conversions between managed and native types. Despite the ability to use the C++ type system on both sides, you often have to convert a native type to a managed type and vice versa. For example, if your function has a native string argument so that native clients can call you, you often have to convert that argument to a managed string so that you can pass it into a managed API. Finally, this chapter discusses how managed classes can use fields of native types and how native types can have data members referring to managed objects.
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CHAPTER 8 MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM
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Even though the C++/CLI compiler silently maps all usages of native types in your source code to functioning IL code, it makes sense to have a closer look at the internals. Let s begin with a simple function: int f(double d); Since all numeric primitives like int and double have counterparts with the same binary layout in the managed type system, the C++/CLI compiler simply considers equivalent managed and native primitives as one type. To the C++/CLI compiler, the following declaration is equivalent to the preceding one: System::Int32 f(System::Double d); However, the Visual C++ compiler supports two 32-bit signed integer types: int and long. As a C++ programmer, you can write the following two overloads of the function f: void f(int i) { /* ... */ } void f(long l) { /* ... */ } Both int and long should be mapped to System::Int32. However, there must be an option to differentiate both functions. To achieve this, the compiler uses a signature modifier. The signature for the function with the int argument just uses the IL keyword for System::Int32, which is int32: .method assembly static void f(int32 i) cil managed The method with the long argument has an int32 argument with a signature modifier to express that the parameter type should be mapped to the native type long instead of int: .method assembly static void f( int32 modopt([mscorlib]System.Runtime.CompilerServices.IsLong) l ) cil managed At first glance, signature modifiers have a lot of similarities to .NET attributes that are applied to a parameter. Both represent metadata that provides extra information about an argument of a function. However, in contrast to attributes, signature modifiers are part of the method s signature and therefore part of the method s identity. The runtime can differentiate between both overloads when they have different signature modifiers, even if the rest of the functions are the same. Managed code can also use functions that have native pointer types as arguments, like the function shown here: void f(int* pi); The C++/CLI compiler translates this function into the following IL method: .method assembly static void f(int32* pi) cil managed
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