C++11 range-based for() loops evaluate once or multiple times? [duplicate]

问题

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Does a C++11 range-based for loop condition get evaluated every cycle? (1 answer)

Closed 6 years ago.

Given this C++11 example code:

for ( const auto &foo : bar() )
{
    // ... do something with foo...
}

Is it guaranteed by the standard that the expression bar() in this example is evaluated only once?

Or could it end up being called at every iteration of the loop?

回答1:

It is evaluated only once. The standard says that the range-based for loop is equivalent to this:

_{§6.5.4 The range-based for statement [stmt.ranged]}

{
  auto && __range = range-init;
  for ( auto __begin = begin-expr,
        __end = end-expr;
        __begin != __end;
        ++__begin ) {
    for-range-declaration = *__begin;
    statement
  }
}

with range-init being equivalent to ( bar() ) in your case (the expression you specify, surrounded by parenthesis). That expression is only evaluated once as you can see.

来源：https://stackoverflow.com/questions/16259574/c11-range-based-for-loops-evaluate-once-or-multiple-times