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// also no longer supported if ( a + b > c )
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The inability of the language to return a class object efficiently was considered a significant weakness of the language One proposed solution was a language extension to name the function's returning class object For example:
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Matrix operator+( const Matrix& m1, const Matrix& m2 ) name result { Matrix result; // return result; }
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The compiler would then rewrite the function internally to take a third reference parameter:
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// internally rewritten function // under proposed language extension void operator+( Matrix &result, const Matrix& m1, const Matrix& m2 ) { // directly compute into result }
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and transform all uses of the function to compute the result directly in the target class object For example,
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Matrix c = a + b;
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becomes transformed internally into
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Matrix c; operator+( c, a, b );
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This name return value extension never became part of the language but the optimization did It was realized that a compiler could recognize the return of the class object and provide the return value transformation without requiring an explicit language extension Given a function of the general form
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classType
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functionName( paramList ) { classType namedResult; // do the work return namedResult; }
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the compiler transforms both the function and all its uses internally to the form
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void functionName( classType &namedResult, paramList ) { // directly compute into namedResult }
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eliminating both the return by value of the class object and the need to invoke the class copy constructor To trigger it, the class object returned must be the same named object at each return point within the function One final point about C++ class object efficiency The initialization of a class object such as
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Matrix c = a + b;
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is always more efficient than its assignment For example, while the program results are exactly the same, writing
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Matrix c; c = a + b;
requires considerably more computation to reach those results Similarly, it is more efficient to write
for ( int ix = 0; ix < size-2; ++ix ) { Matrix matSum = mat[ix] + mat[ix+1]; // }
than
Matrix matSum; for ( int ix = 0; ix < size-2; ++ix ) { matSum = mat[ix] + mat[ix+1]; // }
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file:///F|/WinDDK/resources/CPPPrimer/c++primerhtm
The reason the assignment is always more expensive is that, in general, we cannot substitute directly the target of the assignment for the local object being returned That is, whereas
Point3d p3 = operator+( p1, p2 );
can be safely transformed into
// Pseudo C++ code Point3d p3; operator+( p3, p1, p2 );
it is not safe to transform
Point3d p3; p3 = operator+( p1, p2 );
into
// Pseudo C++ code // not safe in case of an assignment operator+( p3, p1, p2 );
The problem is that the transformed function requires the object passed to it to represent raw storage Why Because the first thing done to the object is to apply the constructor that had been applied to the named local object If the object being passed in has already been constructed, then it is potentially semantically disastrous to construct it a second time An object being initialized is guaranteed to represent raw storage An object being assigned, if the class declares associated constructors (and that is the case we are considering), is guaranteed not to represent raw storage and so cannot be directly passed safely to the function Instead, the compiler must create raw storage in the form of a temporary class object, pass that object to the function, then memberwise assign the temporary object to the target of the assignment Finally, if the class has an associated destructor, it must apply the destructor to the temporary object For example, a likely transformation of
Point3d p3; p3 = operator+( p1, p2 );