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6161 Union Initialization and Assignment As with other complex IDL types, a union has a constructor, a copy constructor, an assignment operator, and a destructor
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The default constructor of a union performs no application-visible initialization of the class This means that you must explicitly initialize the union before reading any of its contents You are not even allowed to read the discriminator value of a defaultconstructed union
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U(const U &) U & operator=(const U &)
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The copy constructor and assignment operator make deep copies, so if a union contains a string, the string contents are copied appropriately
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The destructor destroys a union If the union contains a variable-length member, the memory for that member is deallocated correctly Destroying an uninitialized defaultconstructed union is safe 6162 Union Member and Discriminator Access To activate or assign to a union member, you invoke the corresponding modifier member function Assigning to a union member also sets the discriminator value You can read the discriminator by calling the _d member function For example:
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U my_u; my_ulong_mem(99); assert(my_u_d() == 'L'); assert(my_ulong_mem() == 99); // // // // 'my_u' is not initialized Activate long_mem Verify discriminator Verify value
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In this example, the union is not initialized after default construction Calling the modifier function for the member long_mem initializes the union by activating that member and setting its value As a side effect, assigning to a member via the modifier function also sets the discriminator value The preceding code tests the discriminator value in an assertion to verify that the union works correctly It also reads the value of long_mem by calling its accessor member function Because we just set the value to 99, the accessor must of course return that value The code tests this with another assertion To change the active member of a union, you can use the modifier for a different member to assign to that member:
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my_uchar_mem('X'); // Activate and assign to char_mem
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// Discriminator is now 'c' or 'C', who knows my_u_d('C'); // Now it is definitely 'C'
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Activating the member char_mem sets the discriminator value accordingly The problem in this case is that there are two legal discriminator values: 'c' and 'C' Activating the member char_mem sets the discriminator to one of these two values, but you have no way of knowing which one (the choice is implementation-dependent) The preceding code example explicitly sets the value of the discriminator to 'C' after activating the member You cannot set the discriminator value if that would deactivate or activate a member:
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my_uchar_mem('X'); assert(my_u_d() == my_u_d('c'); my_u_d('C'); my_u_d('X'); // Activate and assign char_mem 'c' || my_u_d() == 'C'); // OK // OK // Illegal, would activate string_mem
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The preceding example shows that you can set the discriminator only to a value that is consistent with the currently active union member (the only legal values here are 'c' and 'C') Setting the discriminator value to anything else results in undefined behavior, and many implementations will deliberately force a core dump to let you know that your program contains a serious run-time error Setting the default member of the union leaves the discriminator in a partially undefined state:
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my_ustring_mem(CORBA::string_dup("Hello")); // Discriminator value is now anything except 'c', 'C', or 'L' assert(my_u_d() != 'c' && my_u_d() != 'C' && my_u_d() != 'L');
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The implementation of the union type picks a discriminator value that is legal for the default member, but, again, the precise value chosen is implementation-dependent This behavior can be inconvenient, for example during tracing Suppose you have trace statements throughout your code that print the discriminator value to the display at various points A problem arises if the default member string_mem is active in the union, because the value of the discriminator can be any character except 'c', 'C', and 'L' This makes it entirely possible for the discriminator to contain non-printable characters, such as a form feed, escape, or Ctrl-S Depending on the display you are using, these characters may cause undesirable effects For example, an escape character can cause the display to clear its screen or switch into block mode, and a Ctrl-S typically acts as a flow-control character that suspends output In general, the default case and multiple case labels for the same union member do not assign a definite value to the discriminator of the union We recommend that you use these IDL features with caution Usually, you can express the desired design in some other way and avoid the potentially awkward coding issues involved
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