C++: Default function arguments & Placeholder arguments

Yao Yao on March 24, 2015
  • Published in category
  • C++

整理自 Thinking in C++

Default arguments

如果是 declare + define 一气呵成,那就这么写:

int foo(int a, int b = 0) {
	return a+b;

如果是先 declare 再 define,那么只需要在 declaration 里写 default arguments;definition 里不用写(写了还算错)。加个注释会是个很好的习惯:

int bar(int a, int b = 0);

int bar(int a, int b /* = 0 */) {
	return a+b;

There are two rules you must be aware of when using default arguments.

  • First, only trailing arguments may be defaulted. That is, you can’t have a default argument followed by a non-default argument.
  • Second, once you start using default arguments in a particular function call, all the subsequent arguments in that function’s argument list must be defaulted (this follows from the first rule).

Placeholder arguments

我们知道,declare function 时可以不写参数名,比如:

int foo(int, int);

Digress:和 Default arguments 一起用就更微妙了。比如:

int foo(int, int = 0);

其实这还不是重点。重点是:definition 里也可以不写参数名!我们称这样的参数为 placeholder arguments,比如:

int foo(int a, int) {
  • Placeholder arguments 不像 default arguments 那样有顺序上的附加要求,参数列表里随便哪个位置上都可以设置 placeholder
  • Placeholder arguments 不能在函数体中使用
    • 没名字自然无法使用
    • 要能使用就不叫 placeholder 了
  • The idea is that you might want to change the function definition to use the placeholder later, without changing all the code where the function is called.
    • Of course, you can accomplish the same thing by using a named argument, but if you define the argument for the function body without using it, most compilers will give you a warning message, assuming you’ve made a logical error.
    • By intentionally leaving the argument name out, you suppress this warning.

An especially important use of default arguments is when you start out with a function with a set of arguments, and after it’s been used for a while you discover you need to add arguments. By defaulting all the new arguments, you ensure that all client code using the previous interface is not disturbed.

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