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// model2h // separation model: // only the template declaration is provided template <typename Type> Type min( Type t1, Type t2 ); // model2C // the template definition export template <typename Type> Type min( Type t1, Type t2 ) { /* */ }
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A program that uses an instantiation of the function template min() need only include the header file before using the instantiation:
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// userC #include "model2h" int i, j; double d = min( i, j ); // ok: use that requires an instantiation
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Even though the template definition for min() is not visible in the file userC, the template instantiation min(int,int) can nonetheless be called within this file However, to make this possible, the template min() had to be defined in a special way Can you see how If you look closely at the file model2C, where the function template min() is defined, you will notice that the keyword export precedes the template definition The template min() is defined to be an exported template The export keyword indicates to the compiler that the template definition might be needed to generate function template instantiations used in other files The compiler must then ensure that the template definition is available when these instantiations are generated We declare an exported function template by having the keyword export precede the template keyword in the definition of the template When a function template is exported, we can use an instantiation of the template in any of the program text files; all we need do is to declare the template before we use it If the export keyword were omitted from the definition of the template min(), the implementation might not instantiate the integer instance of the function template min() and we would not be able to link our program properly Note that some implementations may not require the keyword export to be present Some implementations may support the following language extension: a nonexported function template definition may appear in only one of the program text files; instantiations used in other program text files are nonetheless properly instantiated However, this behavior is an extension Standard C++ requires that users mark the function template definitions as exported if only a declaration of the function template is visible in a program text file before the template is instantiated The export keyword does not need to appear on the template declaration located in the header file The declaration of min() in model2h does not specify the keyword export The keyword could be present in this declaration, but it is not necessary A function template must be defined as an exported template only once in a program Unfortunately, because the compiler processes one file at a time, it cannot ordinarily detect when a function template is defined as exported in more than one program text file If such a situation happens, one of the following behaviors may follow: 1 A link-error may be generated to indicate that the function template is defined in more than one file 2 The compiler may instantiate a function template more than once for the same set of template arguments, causing a link-error because of the duplicate definitions for the function template instantiation 3 The implementation may instantiate the function template using one of the exported function template definitions and ignoring
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the other definitions It is therefore not certain that an error will be generated if more than one definition of an exported function template is provided in our program We must be careful when organizing our programs to place our exported function template definitions in only one of the program text files The separation model allows us to nicely separate the interface of our function templates from their implementations, and it allows us to organize our programs so that the interfaces of our function templates are placed in header files and their implementations are placed in text files However, not all compilers support the separation model, and those that support it do not always support it well To support the separation model, more-sophisticated programming environments are needed, and they are not available on all C++ implementations (Our companion text, Inside the C++ Object Model, describes the template instantiation mechanism supported by one C++ implementation: the Edison Design Group compiler) For the purpose of this book, because our template examples are fairly small and because we want the examples to be easy to compile on many C++ implementations, we will limit ourselves to the use of the inclusion model Explicit Instantiation Declarations When we use the inclusion compilation model, we include the function template definitions in every program text file in which a template instantiation is used We have seen that even though it is unknown exactly where and when the compiler instantiates a function template, the program must behave as if the template is instantiated for a particular set of template arguments only once In reality, some compilers (especially older C++ compilers) instantiate a function template for a particular set of template arguments multiple times Under this model, one of these instantiations is selected as the instantiation to be used by the program (when the program is linked or during some sort of prelink phase) The other instantiations are simply ignored Whether a function template is instantiated only once or multiple times, the program results are not affected, because, in the end, only one template instantiation is used by the program However, the compile-time performance of our program might be greatly affected if function templates are instantiated multiple times If the application is made of a large number of files and if a template is instantiated in all of these files, the time needed to compile the application can increase noticeably The instantiation problems of early compilers made the use of templates difficult To help resolve this, Standard C++ provided explicit instantiation declarations to allow the programmer to take control over when template instantiations take place An explicit instantiation declaration is a declaration in which the template keyword is followed by the declaration for the function template instantiation in which the template arguments are explicitly specified In the following example, an explicit instantiation declaration for sum(int*,int) is provided
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template <typename Type> Type sum( Type op1, int op2 ) { /* */ } // explicit instantiation declaration template int* sum< int* >( int*, int );
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This explicit instantiation declaration is a request to instantiate the template sum () with the template argument int* An explicit instantiation declaration for a given function template instantiation must appear only once in a program The definition for the function template must be provided in the file in which the explicit instantiation declaration appears If this definition is not visible, the explicit instantiation declaration is an error For example:
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