访问者是否会影响应用程序的性能？

Question

I was wondering if the use of accessors can significantly affect performance of an application. Let's say we have a class Point and there are two private fields. We can get access to these fields by calling public functions such as GetX().

class Point
{
public:
    Point(void);
    double GetX();
    double GetY();
    void SetX(double x);
    void SetY(double y);

    ~Point(void);

private:
    double x,y;
};

However if we need to get the value of field xa lot of time (eg if we process images) wouldn't this construction affect the performance of application? Maybe it would be faster just to make fields x and y public?

Answer 1

First and foremost, this is probably premature optimization , and in the general case accessors are not the source of application-level bottlenecks. However, they're not magic pixie dust. It's generally not the case that accessors will hurt performance. There are a few things to consider:

If the implementation is inline or if you have a toolchain that supports link-time optimization, it's likely that there will be 0 impact. Here's an example that lets you get absolutely the same performance on a compiler that doesn't suck.

class Point {
    public: double GetX() const;
    private: double x;
};
inline double Point::GetX() const { return x; }

If the implementation is out-of-line, then you have the added cost of a function call. If, as you say, the function is being called many times, then at least the code is more or less guaranteed to be in the cache, but the relative % of overhead may be high: the work to perform the function call is higher than the work of moving a double around, and there's a pointer indirection because the function actually uses this as a parameter.

If the implementation is both out-of-line and part of a relocatable library (Linux *.so or Windows *.dll), there's an additional indirection that occurs in order to manage the relocation.

Both of the latter costs are reduced on x86-64 hardware relative to x86 32-bit; so much so that you should just not worry about it. I can't speak about other architectures.

Penultimately, if you have many trivial objects with trivial getters and setters, and if you have no profile-guided optimization or link-time optimization, there may be caching effects due to large numbers of tiny functions. It's likely that each function requires a minimum of one cache line, and the functions are not going to be naturally organized in a way that groups commonly-used sections together. This cost is something you should probably ignore unless you're writing a very large-scale C++ project or core component, such as the KDE base system.

Ultimately, don't worry about it.

Answer 2

Such methods should always be inlined by the compiler and the performance of that will be identical to making them public. You can use the inline keyword to help the compiler along, but that's just a hint. If it's really critical that you avoid function call overhead, read the generated assembly. If they're getting inlined you're ok. Otherwise you might want to consider loosening their visibility.

Answer 3

In a typical case, no, there will not be a difference in performance (unless you've fairly specifically told the compiler not to inline any functions). If you allow it to inline functions, however, chances are that it'll generate identical assembly language for both.

That should not , however, be seen as an excuse for ruining your design by including these abominations. First of all, a class should generally provide high level operations, so (for example) you could have a move_relative and move_absolute , so instead of something like this:

Point whatever;

whatever.SetX(GetX()+3);
whatever.SetY(GetY()+4);

...you'd do something like this:

Point whatever;

whatever.move_relative(3, 4);

There are times, however, that exposing something as data really does make sense and work well. If/when you are going to do that, C++ already provides a good way to encapsulate access to the data: a class. It also provides a predefined name for SetXXX and GetXXX -- they're operator= and operator T respectively. The right way to do this is something like this:

template <class T>
class encapsulate {
    T value;
public:
    encapsulate(T const &t) : value(t) {}
    encapsulate &operator=(encapsulate const &t) { value = t.value; }
    operator T() { return value; }
};

Using this, your Point class looks like:

struct Point { 
    encapsulate<double> x, y;
};

With this, the data you want to be public looks and acts as if it is. At the same time, you retain full control over getting/setting the values by changing the encapsulate to something that does whatever you need done.

Point whatever;
whatever.x = whatever.x + 3;
whatever.y = whatever.y + 4;

Though I haven't bothered to in the demo template above, it's fairly easy to support the normal compound assignment operators ( += , -= , *= , /= , etc.) as well. Depending on the situation, it's often useful to eliminate many of these though. Just for example, adding/subtracting to an X/Y coordinate often makes sense -- but multiplication and division frequently won't, so you can just add += and -= , and if somebody accidentally types in /= or |= (for just a couple of examples), their code simply won't compile.

This also provides better enforcement of whatever constraints you need on the data. With private data and an accessor/mutator, other code in the class can (and almost inevitably will) modify the data in ways you didn't want. With a class dedicated to nothing by enforcing the correct constraints, that issue is virtually eliminated. Instead, code both inside and outside the class does a simple assignment (or uses the value, as the case may be) and it's routed through the operator= / operator T automatically -- code inside the class can't bypass whatever checking is needed.

Since you're (apparently) concerned with efficiency, I'll add that this won't normally have any run-time cost either. In fact, being a template gives it a slight advantage in that regard. Where code in a normal function could (even if only by accident) be rewritten in a way that prevented inline expansion, using a template eliminates that -- if you try to rewrite it in a way that otherwise wouldn't generate inline code, with a template it won't compile at all.

Answer 4

As long as you define the functions in the header so the compiler can inline them there should be no difference at all. But even if they aren't inlined you still shouldn't make them public unless profiling indicates that it's a significant bottleneck and that making the variables public improves the problem. Making variables public decreases encapsulation and maintainability. For a bit more on public variables, see my answer on What good are public variables then?

Answer 5

The short answer is yes, this will affect the performance. Whether you will notice the difference or not is another matter that depends on how much code you have in the accessors, among other things.

The more important questions, though, is do you need what you gain from using accessors? If you make the fields public, then you lose control over their values. Do you want to allow x or y to be NaN? or +-infinity? Making them public would make such cases possible.

If you decide later that a double is not acceptable for your point class (maybe you need more precision or the precision isn't necessary), then accessing the fields directly would cause trouble. While this change might also require changes in the accessors, the setters should be fine with overloaded methods. And you may still be fine with a public representation of a double whereas the internal representation is not a double (although this is not so likely with a Point class, I imagine).

There are other cases where you might want to have side effects on accessors and setters as well that making the fields public would circumvent. Maybe you want to create events for when your point changes, but if the fields are public, then your class won't know when the values change.

ADDED

Ok, so my glossing over with my "yes" so that I could get to the non-performance issues that I felt more important wasn't appreciated.

In many cases, the yes is probably as correct as it will be imperceptible. True, using inline and a kick-ass compiler may very well end up with the same code (assuming an accessor like double GetX() { return x; } ), but there are a lot of ifs there. Compilers will only inline things that end up in the same object file (often created from a single code file). So you also need a kick-ass linker to optimize the references in other object files (by the time you get to the linker, the inline hint may not even still remain in the code). So some, but not necessarily all, of the code may end up being identical, but that would be something you can confirm only after the fact and isn't useful.

If you're concerned about image processing then it might be worth allowing for friend classes so that an image class that you code can have access directly to the fields, but again I don't think that even in that case the accessor will be adding a lot to your runtime.