Do repetitive calls to member functions hurt?_问答_开发者

I have programmed in both Java and C, and now I am trying to get my han开发者_如何学JAVAds dirty with C++.

Given this code:

class Booth {

private :
          int tickets_sold;
public :
          int get_tickets_sold();
          void set_tickets_sold();
};

In Java, wherever I needed the value of tickets_sold, I would call the getter repeatedly.

For example:

if (obj.get_tickets_sold() > 50 && obj.get_tickets_sold() < 75){
     //do something
}

In C I would just get the value of the particular variable in the structure:

if( obj_t->tickets_sold > 50 && obj_t->tickets_sold < 75){
    //do something
}

So while using structures in C, I save on the two calls that I would otherwise make in Java, the two getters that is, I am not even sure if those are actual calls or Java somehow inlines those calls.

My point is if I use the same technique that I used in Java in C++ as well, will those two calls to getter member functions cost me, or will the compiler somehow know to inline the code? (thus reducing the overhead of function call altogether?)

Alternatively, am I better off using:

int num_tickets = 0;

if ( (num_tickets = obj.get_ticket_sold()) > 50 && num_tickets < 75){
    //do something
}

I want to write tight code and avoid unnecessary function calls, I would care about this in Java, because, well, we all know why. But, I want my code to be readable and to use the private and public keywords to correctly reflect what is to be done.

Unless your program is too slow, it doesn't really matter. In 99.9999% of code, the overhead of a function call is insignificant. Write the clearest, easiest to maintain, easiest to understand code that you can and only start tweaking for performance after you know where your performance hot spots are, if you have any at all.

That said, modern C++ compilers (and some linkers) can and will inline functions, especially simple functions like this one.

If you're just learning the language, you really shouldn't worry about this. Consider it fast enough until proven otherwise. That said, there are a lot of misleading or incomplete answers here, so for the record I'll flesh out a few of the subtler implications. Consider your class:

class Booth
{
  public:
    int get_tickets_sold();
    void set_tickets_sold();
  private:
    int tickets_sold;
};

The implementation (known as a definition) of the get and set functions is not yet specified. If you'd specified function bodies inside the class declaration then the compiler would consider you to have implicitly requested they be inlined (but may ignore that if they're excessively large). If you specify them later using the inline keyword, that has exactly the safe effect. Summarily...

class Booth
{
  public:
    int get_tickets_sold() { return tickets_sold; }
    ...

...and...

class Booth
{
  public:
    int get_tickets_sold();
    ...
};

inline int Booth::get_tickets_sold() { return tickets_sold; }

...are equivalent (at least in terms of what the Standard encourages us to expect, but individual compiler heuristics may vary - inlining is a request that the compiler's free to ignore).

If the function bodies are specified later without the inline keyword, then the compiler is under no obligation to inline them, but may still choose to do so. It's much more likely to do so if they appear in the same translation unit (i.e. in the .cc/.cpp/.c++/etc. "implementation" file you're compiling or some header directly or indirectly included by it). If the implementation is only available at link time then the functions may not be inlined at all, but it depends on the way your particular compiler and linker interact and cooperate. It is not simply a matter of enabling optimisation and expecting magic. To prove this, consider the following code:

// inline.h:
void f();

// inline.cc:
#include <cstdio>
void f() { printf("f()\n"); }

// inline_app.cc:
#include "inline.h"
int main() { f(); }

Building this:

g++ -O4 -c inline.cc
g++ -O4 -o inline_app inline_app.cc inline.o

Investigating the inlining:

$ gdb inline_app 
...
(gdb) break main
Breakpoint 1 at 0x80483f3
(gdb) break f
Breakpoint 2 at 0x8048416
(gdb) run
Starting program: /home/delroton/dev/inline_app 

Breakpoint 1, 0x080483f3 in main ()
(gdb) next
Single stepping until exit from function main, 
which has no line number information.

Breakpoint 2, 0x08048416 in f ()
(gdb) step
Single stepping until exit from function _Z1fv, 
which has no line number information.
f()
0x080483fb in main ()
(gdb)

Notice the execution went from 0x080483f3 in main() to 0x08048416 in f() then back to 0x080483fb in main()... clearly not inlined. This illustrates that inlining can't be expected just because a function's implementation is trivial.

Notice that this example is with static linking of object files. Clearly, if you use library files you may actually want to avoid inlining of the functions specifically so that you can update the library without having to recompile the client code. It's even more useful for shared libraries where the linking is done implicitly at load time anyway.

Very often, classes providing trivial functions use the two forms of expected-inlined function definitions (i.e. inside class or with inline keyword) if those functions can be expected to be called inside any performance-critical loops, but the countering consideration is that by inlining a function you force client code to be recompiled (relatively slow, possibly no automated trigger) and relinked (fast, for shared libraries happens on next execution), rather than just relinked, in order to pick up changes to the function implementation.

These kind of considerations are annoying, but deliberate management of these tradeoffs is what allows enterprise use of C and C++ to scale to tens and hundreds of millions of lines and thousands of individual projects, all sharing various libraries over decades.

One other small detail: as a ballpark figure, an out-of-line get/set function is typically about an order of magnitude (10x) slower than the equivalent inlined code. That will obviously vary with CPU, compiler, optimisation level, variable type, cache hits/misses etc..

No, repetitive calls to member functions will not hurt.

If it's just a getter function, it will almost certainly be inlined by the C++ compiler (at least with release/optimized builds) and the Java Virtual Machine may "figure out" that a certain function is being called frequently and optimize for that. So there's pretty much no performance penalty for using functions in general.

You should always code for readability first. Of course, that's not to say that you should completely ignore performance outright, but if performance is unacceptable then you can always profile your code and see where the slowest parts are.

Also, by restricting access to the tickets_sold variable behind getter functions, you can pretty much guarantee that the only code that can modify the tickets_sold variable to member functions of Booth. This allows you to enforce invariants in program behavior.

For example, tickets_sold is obviously not going to be a negative value. That is an invariant of the structure. You can enforce that invariant by making tickets_sold private and making sure your member functions do not violate that invariant. The Booth class makes tickets_sold available as a "read-only data member" via a getter function to everyone else and still preserves the invariant.

Making it a public variable means that anybody can go and trample over the data in tickets_sold, which basically completely destroys your ability to enforce any invariants on tickets_sold. Which makes it possible for someone to write a negative number into tickets_sold, which is of course nonsensical.

The compiler is very likely to inline function calls like this.

class Booth {
public:
    int get_tickets_sold() const { return tickets_sold; }

private:
    int tickets_sold;
};

Your compiler should inline get_tickets_sold, I would be very surprised if it didn't. If not, you either need to use a new compiler or turn on optimizations.

Any compiler worth its salt will easily optimize the getters into direct member access. The only times that won't happen are when you have optimization explicitly disabled (e.g. for a debug build) or if you're using a brain-dead compiler (in which case, you should seriously consider ditching it for a real compiler).

The compiler will very likely do the work for you, but in general, for things like this I would approach it more from the C perspective rather than the Java perspective unless you want to make the member access a const reference. However, when dealing with integers, there's usually little value in using a const reference over a copy (at least in 32 bit environments since both are 4 bytes), so your example isn't really a good one here... Perhaps this may illustrate why you would use a getter/setter in C++:

class StringHolder
{
public:
  const std::string& get_string() { return my_string; }
  void set_string(const std::string& val) { if(!val.empty()) { my_string = val; } }
private
  std::string my_string;
}

That prevents modification except through the setter which would then allow you to perform extra logic. However, in a simple class such as this, the value of this model is nil, you've just made the coder who is calling it type more and haven't really added any value. For such a class, I wouldn't have a getter/setter model.