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Do s and Don ts of Returning Values
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Handles may be used as return values, just as pointers can be. Tracking references may also be used as return values, as long as you take care not to return a reference to a temporary variable. Objects that are to be destroyed at the end of a function call, such as reference types declared using stack semantics, should not be returned as a tracking reference, since they will be destroyed when the function scope ends. If the return type is an object type (not a handle), then it must have a copy constructor. Listing 4-29 illustrates several return value scenarios.
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CHAPTER 4 OBJECT SEMANTICS IN C++/CLI
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Listing 4-29. Returning Values // return_values.cpp using namespace System; ref class R { bool destroyed; public: R() { } R(const R% r) { }
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// copy constructor
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R% GetTrackingRefMF(); void PrintR() { if (destroyed) Console::WriteLine("Using destroyed object!"); else Console::WriteLine("R"); } ~R() { destroyed = true; } }; value struct V { int a; int b; }; // Handle return value: OK R^ GetHandle() { // Create a new R. R^ r = gcnew R(); // Return it. return r; } // Return reference to local variable. // -- avoid R% GetTrackingRef() { // Create a new R. R^ r = gcnew R(); return *r; // compiler warning }
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CHAPTER 4 OBJECT SEMANTICS IN C++/CLI
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// Return reference to local variable. // -- avoid R% GetTrackingRef_Bad() { R r; return r; // compiler warning } // OK: return a nontemporary reference. R% R::GetTrackingRefMF() { return *this; } // Value type return value: OK V GetValue() { V v; v.a = 100; v.b = 54; // Value gets copied. return v; } // Return value with stack semantics. // Requires copy constructor R GetR() { R r; return r; // requires copy constructor }
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int main() { // Valid uses: R^ r1 = GetHandle(); // OK R% r2 = r1->GetTrackingRefMF(); // OK V v1 = GetValue(); // OK Console::WriteLine("{0} {1}", v1.a, v1.b); R r3 = GetR(); // OK only if R has a copy constructor
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CHAPTER 4 OBJECT SEMANTICS IN C++/CLI
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// Using a tracking reference in the GetTrackingRef function works, // but a handle would work as well and would eliminate the compiler // warning in the function declaration. R% r4 = GetTrackingRef(); r4.PrintR(); // Using the tracking reference here is not OK // since the destructor was called. R% r5 = GetTrackingRef_Bad(); r5.PrintR(); } The output of Listing 4-29 is shown here: 100 54 R Using destroyed object! This code illustrates several return value possibilities using handles, references, and value types. Just as it is not a good idea to return references to local variables in classic C++, returning tracking references to local variables is not a good idea in C++/CLI since the destructor is called for local variables, leaving the caller with a reference to a destructed object. Instead, create a permanent object (with gcnew) and return a handle. This is particularly important for C# programmers since C# allows references to be returned from functions. This works in C# since C# doesn t have the concept of stack semantics for reference types.
Summary
In this chapter, you looked at reference types and value types and at the many different ways of referring to objects in code. You saw the semantic differences between these methods, including objects with heap and stack semantics, tracking references, dereferencing handles, copying objects, gc-lvalues, and the auto_handle template. You also looked at passing parameters in various ways, the behavior of C++/CLI handles and tracking references as function parameters, and using handles and references as return values. Now you ll learn about some fundamental, but special, types: strings, arrays, and enums.
Fundamental Types: Strings, Arrays, and Enums
n this chapter, you ll learn about some special types in the C++/CLI type system. I have been using the term primitive type to refer to the built-in integral and floating-point types. Other types, such as those discussed in this chapter, are built upon these primitive types and are fundamental to any program. Each of these types is a .NET version of a classic C++ concept, and each of these has special language support in addition to being a bona fide .NET Framework object type. The chapter will go into some detail not just on the syntax and mechanics of the types themselves, but also some of the commonly used .NET Framework library functionality related to these types. My primary aim in this book is to focus on the C++/CLI language itself, not the .NET Framework. However, input and output is so fundamental to any language that it s worth discussing on its own, and what better place to discuss it than in the context of strings Input and output of text are necessary for almost any application, not just an old-style console application. You might need to output text to a string for display in a user interface or for a formatted file. Output usually involves manipulating strings, so this chapter first looks in depth at the String type. The String type is the one that actually provides much of the formatting capability needed in output, whether it s to the console or a web application or a graphical user interface.
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