“const T＆arg”与“T arg”

Question

Which of the following examples is the better way of declaring the following function and why?

void myFunction (const int &myArgument);

or

void myFunction (int myArgument);

Answer 1

如果sizeof(T)>sizeof(void*) ，则使用const T & arg ，如果sizeof(T) <= sizeof(void*) ，则使用T arg

Answer 2

They do different things. const T& makes the function take a reference to the variable. On the other hand, T arg will call the copy constructor of the object and passes the copy . If the copy constructor is not accessible (eg it's private ), T arg won't work:

class Demo {
    public: Demo() {} 
    private: Demo(const Demo& t) { } 
};

void foo(Demo t) { }

int main() {
    Demo t;
    foo(t); // error: cannot copy `t`.
    return 0;
}

For small values like primitive types (where all matters is the contents of the object, not the actual referential identity; say, it's not a handle or something), T arg is generally preferred. For large objects and objects that you can't copy and/or preserving referential identity is important (regardless of the size), passing the reference is preferred.

Another advantage of T arg is that since it's a copy, the callee cannot maliciously alter the original value. It can freely mutate the variable like any local variables to do its work.

Answer 3

Taken from Move constructors . I like the easy rules

1. If the function intends to change the argument as a side effect, take it by reference/pointer to a non-const object. Example:

    void Transmogrify(Widget& toChange); void Increment(int* pToBump); 

2. If the function doesn't modify its argument and the argument is of primitive type, take it by value. Example:

    double Cube(double value); 

3. Otherwise

   3.1. If the function always makes a copy of its argument inside, take it by value.

   3.2. If the function never makes a copy of its argument, take it by reference to const.

   3.3. Added by me : If the function sometimes makes a copy, then decide on gut feeling: If the copy is done almost always, then take by value. If the copy is done half of the time, go the safe way and take by reference to const.

In your case, you should take the int by value, because you don't intend to modify the argument, and the argument is of primitive type. I think of "primitive type" as either a non-class type or a type without a user defined copy constructor and where sizeof(T) is only a couple of bytes.

Answer 4

There's a popular advice that states that the method of passing ("by value" vs "by const reference") should be chosen depending in the actual size of the type you are going to pass. Even in this discussion you have an answer labeled as "correct" that suggests exactly that.

In reality, basing your decision on the size of the type is not only incorrect, this is a major and rather blatant design error, revealing a serious lack of intuition/understanding of good programming practices.

Decisions based on the actual implementation-dependent physical sizes of the objects must be left to the compiler as often as possible. Trying to "tailor" your code to these sizes by hard-coding the passing method is a completely counterproductive waste of effort in 99 cases out of 100. (Yes, it is true, that in case of C++ language, the compiler doesn't have enough freedom to use these methods interchangeably - they are not really interchangeable in C++ in general case. Although, if necessary, a proper size-based [semi-]automatic passing methios selection might be implemented through template metaprogramming; but that's a different story).

The much more meaningful criterion for selecting the passing method when you write the code "by hand" might sound as follows:

1. Prefer to pass "by value" when you are passing an atomic, unitary, indivisible entity, such as a single non-aggregate value of any type - a number, a pointer, an iterator. Note that, for example, iterators are unitary values at the logical level. So, prefer to pass iterators by value , regardless of whether their actual size is greater than sizeof(void*). (STL implementation does exactly that, BTW).

2. Prefer to pass "by const reference" when you are passing an aggregate, compound value of any kind. ie a value that has exposed pronouncedly "compound" nature at the logical level, even if its size is no greater than sizeof(void*).

The separation between the two is not always clear, but that how things always are with all such recommendations. Moreover, the separation into "atomic" and "compound" entities might depend on the specifics of your design, so the decision might actually differ from one design to the other.

Note, that this rule might produce decisions different from those of the allegedly "correct" size-based method mentioned in this discussion.

As an example, it is interesing to observe, that the size-based method will suggest you manually hard-code different passing methods for different kinds of iterators, depending on their physical size. This makes is especially obvious how bogus the size-based method is.

Once again, one of the basic principles from which good programming practices derive, is to avoid basing your decisions on physical characteristics of the platform (as much as possible). Instead, you decisions have to be based on the logical and conceptual properties of the entities in your program (as much as possible). The issue of passing "by value" or "by reference" is no exception here.

---

In C++11 introduction of move semantics into the language produced a notable shift in the relative priorities of different parameter-passing methods. Under certain circumstances it might become perfectly feasible to pass even complex objects by value

Should all/most setter functions in C++11 be written as function templates accepting universal references?

Answer 5

Contrary to popular and long-held beliefs, passing by const reference isn't necessarily faster even when you're passing a large object. You might want to read Dave Abrahams recent article on this very subject.

Edit: (mostly in response to Jeff Hardy's comments): It's true that passing by const reference is probably the "safest" alternative under the largest number of circumstances -- but that doesn't mean it's always the best thing to do. But, to understand what's being discussed here, you really do need to read Dave's entire article quite carefully, as it is fairly technical, and the reasoning behind its conclusions is not always intuitively obvious (and you need to understand the reasoning to make intelligent choices).

Answer 6

Usually for built-in types you can just pass by value. They're small types.

For user defined types (or templates, when you don't what is going to be passed) prefer const&. The size of a reference is probably smaller than the size of the type. And it won't incurr an extra copy (no call to a copy constructor).

Answer 7

Well, yes ... the other answers about efficiency are true. But there's something else going on here which is important - passing a class by value creates a copy and, therefore, invokes the copy constructor. If you're doing fancy stuff there, it's another reason to use references.

Answer 8

A reference to const T is not worth the typing effort in case of scalar types like int, double, etc. The rule of thumb is that class-types should be accepted via ref-to-const. But for iterators (which could be class-types) we often make an exception.

In generic code you should probably write "T const&" most of the time to be on the safe side. There's also boost's call traits you can use to select the most promising parameter passing type. It basically uses ref-to-const for class types and pass-by-value for scalar types as far as I can tell.

But there are also situations where you might want to accept parameters by value, regardless of how expensive creating a copy can be. See Dave's article "Want Speed? Use pass by value!" .

Answer 9

For simple types like int, double and char*, it makes sense to pass it by value. For more complex types, I use const T& unless there is a specific reason not to.

The cost of passing a 4 - 8 byte parameter is as low as you can get. You don't buy anything by passing a reference. For larger types, passing them by value can be expensive.

Answer 10

It won't make any difference for an int, as when you use a reference the memory address still has to be passed, and the memory address (void*) is usually about the size of an integer.

For types that contain a lot of data it becomes far more efficient as it avoids the huge overhead from having to copy the data.

Answer 11

Well the difference between the two doesn't really mean much for ints.

However, when using larger structures (or objects), the first method you used, pass by const reference, gives you access to the structure without need to copy it. The second case pass by value will instantiate a new structure that will have the same value as the argument.

In both cases you see this in the caller

myFunct(item);

To the caller, item will not be changed by myFunct, but the pass by reference will not incur the cost of creating a copy.

There is a very good answer to a similar question over at Pass by Reference / Value in C++

Answer 12

The difference between them is that one passes an int (which gets copied), and one uses the existing int. Since it's a const reference, it doesn't get changed, so it works much the same. The big difference here is that the function can alter the value of the int locally, but not the const reference. (I suppose some idiot could do the same thing with const_cast<> , or at least try to.) For larger objects, I can think of two differences.

First, some objects simply can't get copied, auto_ptr<> s and objects containing them being the obvious example.

Second, for large and complicated objects it's faster to pass by const reference than to copy. It's usually not a big deal, but passing objects by const reference is a useful habit to get into.

Answer 13

Either works fine. Don't waste your time worrying about this stuff.

The only time it might make a difference is when the type is a large struct, which might be expensive to pass on the stack. In that case, passing the arg as a pointer or a reference is (slightly) more efficient.

Answer 14

The problem appears when you are passing objects. If you pass by value, the copy constructor will be called. If you haven't implemented one, then a shallow copy of that object will be passed to the function.

Why is this a problem? If you have pointers to dynamically allocated memory, this could be freed when the destructor of the copy is called (when the object leaves the function's scope). Then, when you re call your destructor, youll have a double free.

Moral: Write your copy constructors.