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We can now understand what happens in this expression As we know, the if tests a condition, which is an expression that yields a truth value The precise type of such a truth value is bool Using a value of any arithmetic or pointer type automatically converts the value to type bool, so we can use values of these types as the expression in a condition Of course, iostream is neither a pointer nor an arithmetic type However, the standard library defines a conversion from type istream to void*, which is a pointer to void It does so by defining istream::operator void*, which tests various status flags to determine whether the istream is valid, and returns either 0 or an implementation-defined nonzero void* value to indicate the state of the stream We have not previously used the void* type We said in 622/114 that the void type could be used only in a few
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ways the basis for a pointer being one of them A pointer to void is sometimes called a universal pointer, because it is a pointer that can point to any type of object Of course, you cannot dereference the pointer, because the type of the object to yield isn't known But one thing that can be done with a void* is to convert it to bool, which is exactly how it is used in this context The reason that class istream defines operator void* rather than operator bool is to allow the compiler to detect the following erroneous usage:
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int x; cin << x; // we should have written cin >> x;
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If class istream were to define operator bool, this expression would use istream::operator bool to convert cin to bool, and then convert the resulting bool value to int, shift that value left by a number of bits equal to the value of x, and throw the result away! By defining a conversion to void*, rather than to an arithmetic type, the standard library still allows an istream to be used as a condition, but prevents it from being used as an arithmetic value
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126 Conversions and memory management
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Many C++ programs interface to systems written in C or assembly language that use null-terminated arrays of characters to hold string data As we saw in 1052/180, the C++ standard library itself uses this convention to obtain the names of input and output files Because of this convention, we might conclude that our Str class should provide a conversion from Str to a null-terminated array of characters If we did so, then our users could (automatically) pass Strs to functions that operate on null-terminated arrays Unfortunately, as we shall see, doing so is fraught with memory-management pitfalls Assuming that we wanted to provide a conversion from Str to char*, we'd probably want to provide both const and nonconst versions:
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class Str { public: // plausible, but problematic conversion operations operator char*(); // added operator const char*() const; // added // as before private: Vec<char> data; };
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With this rewrite, Str users could write code such as
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// wishful thinking: convert s and then open the stream named s
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The only problem is that these conversions are almost impossible to implement well We can't just return data, most obviously because it's the wrong type: data is a Vec<char>, and we need an array of char More subtly, even if the types matched, returning data would violate class Str's encapsulation: A user who obtained a pointer to data could use that pointer to change the value of the string Just as bad, consider what happens when the Str is destroyed If the user tries to use the pointer after the Str object no longer exists, then the pointer will point to memory that has been returned to the system and is no longer valid We can solve the encapsulation problem by providing only a conversion to const char*, but doing so does not prevent a user from destroying the Str and then using the pointer We can solve this second problem by allocating new space for a copy of the characters from data, and returning a pointer to this newly allocated space The user would then have to manage this space, freeing it when it is no longer needed As it turns out, this design won't work either Conversions may happen implicitly, in which case the user has no pointer to destroy! Look again at
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