C++ vector<int>& 的默认参数？
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C++ Default argument for vector<int>&?
提问by voteforpedro
I have a function,
我有一个功能，
void test( vector<int>& vec );
How can I set the default argument for vec ? I have tried
如何为 vec 设置默认参数？我试过了
void test( vector<int>& vec = vector<int>() );
But there's a warning "nonstandard extension used : 'default argument' : conversion from 'std::vector<_Ty>' to 'std::vector<_Ty> &'"
但是有一个警告“使用了非标准扩展：‘默认参数’：从‘std::vector<_Ty>’转换为‘std::vector<_Ty> &’”
Is there a better way to do this ? Instead of
有一个更好的方法吗 ？代替
void test() {
vector<int> dummy;
test( dummy );
}
Regards, Voteforpedro
问候， Voteforpedro
回答by ereOn
Have you tried:
你有没有尝试过：
void test(const vector<int>& vec = vector<int>());
C++does not allow temporaries to be bound to non-const references.
C++不允许临时对象绑定到非常量引用。
If you really to need to have a vector<int>&(not a constone), you can declare a static instance and use it as a default (thus non-temporary) value.
如果您真的需要一个vector<int>&（不是const一个），您可以声明一个静态实例并将其用作默认（因此非临时）值。
static vector<int> DEFAULT_VECTOR;
void test(vector<int>& vec = DEFAULT_VECTOR);
But beware, because DEFAULT_VECTOR will (can) be modified and won't reset on each call ! Not sure that this is what you really want.
但要注意，因为 DEFAULT_VECTOR 将（可以）被修改并且不会在每次调用时重置！不确定这是否是您真正想要的。
Thanks to stinky472, here is a thread-safe alternative:
感谢stinky472，这是一个线程安全的替代方案：
Instead of providing a default value, you might as well overload test()with a zero-parameter version which calls the other version:
与其提供默认值，不如test()使用调用另一个版本的零参数版本进行重载：
void test()
{
vector<int> vec;
test(vec);
}
回答by sbi
I find it questionable for a non-constreference argument to have a default value. What is this supposed to mean?
我发现非const引用参数具有默认值是有问题的。这是什么意思？
Commonly, a function taking a non-constreference means "I might change the argument". If the argument is optionalwhy no pass a (non-const) pointer? That way, when callers don't want to pass an arguments, they can pass NULL.
(See herefor more information on how to pass function arguments.)
通常，采用非const引用的函数意味着“我可能会更改参数”。如果参数是可选的，为什么不传递（非const）指针？这样，当调用者不想传递参数时，他们可以传递NULL.
（有关如何传递函数参数的更多信息，请参见此处。）
Edit:Well, and of course, there's also overloading. Just add another overload which doesn't take this argument.
编辑：嗯，当然，还有重载. 只需添加另一个不接受此参数的重载。
回答by stinky472
We cannot initialize mutable references to temporaries. Only const references allow this. What you are after is most likely this:
我们无法初始化对临时对象的可变引用。只有 const 引用允许这样做。你所追求的很可能是这样的：
void test( const vector<int>& vec = vector<int>() );
Aside from avoiding undefined behavior, this makes the most sense logically and from a const-correctness perspective. If you wanted 'test' to modify the original vector being passed to it, you would not have been able to sensibly provide a default value. Thus it's obvious you are using 'vec' here for read-only purposes and should therefore make it a const reference.
除了避免未定义的行为之外，这在逻辑上和从常量正确性的角度来看是最有意义的。如果您希望 'test' 修改传递给它的原始向量，您将无法明智地提供默认值。因此，很明显，您在此处将 'vec' 用于只读目的，因此应将其设为 const 引用。
回答by JohnJohn
I know this question was asked in 2010 but for C++11 you can do it like this:
我知道这个问题是在 2010 年提出的，但对于 C++11，你可以这样做：
void test( vector<int>& vec = {}) {
// Awesome code here
}
As pointed out in this answer.
正如在这个答案中指出的那样。
回答by Ahmed Hamdy
You could just make pass a pointer to a vector and initialize it to NULL
您可以只传递一个指向向量的指针并将其初始化为 NULL
void test( vector<int>* vec = NULL );
