TAILIEUCHUNG - Thinking in C plus plus (P11)

Tham khảo tài liệu 'thinking in c plus plus (p11)', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | defined types by value during function calls. It s so important in fact that the compiler will automatically synthesize a copyconstructor if you don t provide one yourself as you will see. Passing returning by value To understand the need for the copy-constructor consider the way C handles passing and returning variables by value during function calls. If you declare a function and make a function call int f int x char c int g f a b how does the compiler know how to pass and return those variables It just knows The range of the types it must deal with is so small - char int float double and their variations - that this information is built into the compiler. If you figure out how to generate assembly code with your compiler and determine the statements generated by the function call to f you ll get the equivalent of push b push a call f add sp 4 mov g register a This code has been cleaned up significantly to make it generic the expressions for b and a will be different depending on whether the variables are global in which case they will be _b and _a or local the compiler will index them off the stack pointer . This is also true for the expression for g. The appearance of the call to f will depend on your name-decoration scheme and register a depends on how the CPU registers are named within your assembler. The logic behind the code however will remain the same. In C and C arguments are first pushed on the stack from right to left then the function call is made. The calling code is responsible 480 Thinking in C www. BruceEckel .com for cleaning the arguments off the stack which accounts for the add sp 4 . But notice that to pass the arguments by value the compiler simply pushes copies on the stack - it knows how big they are and that pushing those arguments makes accurate copies of them. The return value of f is placed in a register. Again the compiler knows everything there is to know about the return value type because that type is built into the language so the

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