CODEWITHSUNDEEP
c++python
Tommy
Tommy

Reputation: 13672

Is there an equivalent of Python's `pass` in c++ std11?

I want a statement that does nothing but can be used in places requiring a statement. Pass: http://docs.python.org/release/2.5.2/ref/pass.html

Edit: Just saw: How does one execute a no-op in C/C++?

#define pass (void)0

Solved my problem. Thanks!

Upvotes: 43

Views: 117720

Answers (7)

Prashant Kumar
Prashant Kumar

Reputation: 22629

A null statement (just Semicolon), or empty brackets should work for you

For example Python's

while some_condition():    # presumably one that eventually turns false
    pass

Could translate to the following C++

while (/* some condition */)
    ;

Or

while (/* some condition */) {}

Perhaps for the ternary operator case, you could do:

x > y ? do_something() : true;

Upvotes: 57

Doot
Doot

Reputation: 745

void f() { ; }
void g() { }
void h() { __asm__("nop"); }

all result in almost identical assembly output (x86 64).

f & g both give

push    rbp
mov     rbp, rsp
nop
pop     rbp
ret

while g gives

push    rbp
mov     rbp, rsp
nop
nop
pop     rbp
ret

(one extra nop)

In my testing, it seems that a loop containing just __asm__("nop") takes ~50% longer than ;

When using the -O1 flag or above, the same still applies - f & g become

ret

and h becomes

nop
ret

When using a loop, e.g. for(;;){}, the __asm__ still adds one extra instruction, which is understandable

Upvotes: 1

ibrakap
ibrakap

Reputation: 51

I know it's too absurd but you may think using "nop" instruction.

In Linux

void pass()
{
    __asm__("nop");
}

In Windows

void pass()
{
    __asm{nop};
}

Upvotes: 5

slaughter98
slaughter98

Reputation: 1883

As @bboonn suggests:

 if (some_flag)
   ; // Do nothing

 else if (some_other_flag)
   do_something();

Upvotes: 3

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385295

As has been stated in the comments, this is not supported because it makes no sense. The conditional operator is designed to evaluate to one of two operands. Two. Not one.

It is not okay to abuse the operator to perform some conditional action in only one of those cases. In fact, it is best that neither operand have any side-effects whatsoever. This is not a "do something" construct, but a "give me one of two things" construct.

In this regard, if Python were to support what you say it supports, then it would be broken where C++ is not. As it happens, Python doesn't actually support it either, after all.

Write an if statement, instead:

if (x > y) {
   do_something();
}
else {
   /* Unimplemented at the moment */
}

Upvotes: 5

bboonn
bboonn

Reputation: 31

I think in C++ just an empty line (;) will be the equivalent of 'pass'

Upvotes: 3

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361642

No. You don't have pass or equivalent keyword. But you can write equivalent code without any such keyword.

def f():
   pass

becomes

void f() {}

and 

 class C:
     pass

becomes

 class C {};

In different context, different syntax could be useful. For example,

 class MyError(Exception):
        pass

becomes

class MyError : public std::exception
{
      using std::exception::exception; //inherits constructor!
};

As you can see, in this context, you've to write using to inherits constructors from the base class. In Python, pass does the similar thing, in similar context.

Hope that helps.

Upvotes: 12

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