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IT-SC book: Advanced CORBA Programming with C++
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return 0;
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With this definition of print_string, things are well behaved When the actual parameter is of type String_var, the compiler uses the const char * conversion operator to make the call The conversion operator returns the private pointer inside the String_var and is typically inlined, and that keeps the cost of the call to a minimum Passing the string literal "World" to print_string does not create problems The literal is simply passed as a const char * to the function No temporary is created in either case, and no calls to the memory allocator are necessary 6103 Passing Strings as Parameters for Update Access To pass a string either as a char * or as a String_var to a function for update, a formal parameter of type String_var & will not work If you pass a char * where a String_var & is expected, the compiler creates a temporary This results in construction of a String_var from a char * literal and eventually causes a core dump To get it right, we must use a formal argument type of char * &:
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void update_string(char * & s) { CORBA::string_free(s); s = CORBA::string_dup("New string"); } int main() { CORBA::String_var sv = CORBA::string_dup("Hello"); update_string(sv); cout < sv < endl; // Works fine, prints "New string" char * p = CORBA::string_dup("Hello"); update_string(p); cout < p < endl; // Fine too, prints "New string" CORBA::string_free(p); return 0; }
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A final warning: update_string assumes that the string it is passed was allocated with string_alloc or string_dup This means that the following code is not portable:
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char * p = new char[sizeof("Hello")]; strcpy(p, "Hello"); update_string(p); // Bad news! delete[] p;
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IT-SC book: Advanced CORBA Programming with C++
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This code causes a string allocated by new[] to be deallocated by string_free and causes a string allocated by string_dup to be deallocated by delete[], and that simply does not work on some platforms Calling update_string with an uninitialized pointer is also asking for trouble, because it results in passing a stray pointer to string_free, most likely with disastrous consequences However, passing a variable initialized to null is safe; string_free does nothing when given a null pointer 6104 Problems with Implicit Type Conversions Passing a String_var where a char * is expected relies on implicit type conversion Some compilers do not correctly apply conversion operators, or they incorrectly complain about ambiguous calls Rather than expect every C++ compiler to be perfect, the C++ mapping provides member functions that allow you to perform explicit conversions These member functions are in, inout, out, and _retn (the names suggest the use for passing a parameter in the corresponding direction)
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const char * in() const
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You can call this conversion function if your compiler rejects an attempt to pass a String_var where a const char * is expected For example:
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void print_string(const char * s) { /* */ } // As before // CORBA::String_var sv(CORBA::string_dup("Hello")); print_string(sv); // Assume compiler bug prevents this print_string(svin()); // Explicit call avoids compiler bug
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The in member function returns the private pointer held by the String_var wrapper as a const char * You could achieve the same thing by using a cast:
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print_string((const char *)sv);
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This code explicitly invokes operator const char * on the String_var However, using the in member function is safer than a "sledge-hammer" cast that bypasses all type checking Similar arguments apply to using the inout and out member functions in preference to a cast
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char * & inout()
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You can call the inout member function if your compiler refuses to accept a String_var where a char * & is expected For example:
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void update_string(char * & s) { /* */ } // As before // CORBA::String_var sv; update_string(sv); update_string(svinout()); // Assume compiler bug prevents this // Explicit call avoids compiler bug
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The inout member function returns a reference to the pointer held by the String_var wrapper so that it can be changed (for example, by reallocation)
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char * & out()
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This conversion operator allows you to pass a String_var as an output parameter where a char * & is expected The out member function differs from the inout member function in that out deallocates the string before returning a reference to a null pointer To see why this is necessary, consider the following helper function:
void read_string(char * & s) // s is an out parameter { // Read a line of text from a file s = CORBA::string_dup(line_of_text); }
The caller can use read_string as follows without causing a memory leak:
CORBA::String_var line; read_string(lineout()); read_string(lineout()); cout < line < endl; // Skip first line // Read second line - no memory leak // Print second line
Calling the out member function does two things: it first deallocates whatever string is currently held by the String_var, and then it returns a reference to a null pointer This behavior allows the caller to call read_string twice in a row without creating a memory leak At the same time, read_string need not (but can) deallocate the string before allocating a new value (If it deallocates the string, no harm is done because deallocation of a null pointer is safe) 6105 Yielding Ownership of a String The _retn member function returns the pointer held by a String_var and also yields ownership of the string This behavior is useful if a function must return a dynamically allocated string and also must worry about error conditions For example, consider a get_line helper function that reads a line of text from a database The caller uses the function this way: