C ++中基于范围的for循环的替代方法

Question

Is there any alternative to a range-based for loop when it comes to vector arrays? I've noticed that c++98 won't allow range-based for loops. Here is some sample code looping through a vector array using a range based for loop:

#include <iostream>
#include <vector>

using namespace std;

int main() {
  vector<int> vecArray1 (3,20);
  for (int v: vecArray1) {
    cout << "ArrayValue:" << v << endl;
  }
  return 0;
}

Now here is an alternative I've tried that didn't work:

#include <iostream>
#include <vector>

using namespace std;

int main() {
  vector<int> vecArray1 (3,20);
  for (int i=0; i<sizeof(vecArray1); ++i) {
    cout << "ArrayValue:" << vecArray1[i] << endl;
  }
  return 0;
}

It outputs 10 elements instead of 3 defined by the vector array above. Are there any alternatives to range-based for loops that work with c++98?

Answer 1

sizeof(vecArray1) doesn't do what you think it does.

Either use:

for (int i=0; i<vecArray1.size(); ++i) {
    cout << "ArrayValue:" << vecArray1[i] << endl;
}

or:

for (std::vector<int>::iterator it = vecArray1.begin(); it != vecArray1.end(); it++) {
    cout << "ArrayValue: " << *it << endl;
}

The second one is more verbose, but works for other types of collections as well (like std::list ) and is more similar to what the range-based for loop does.

Answer 2

C++98 does not allow for range-based for-loops. In C++98 you would need to do the following:

for(unsigned int i=0;i<vecArray.size();++i)
    std::cout << "ArrayValue: " << vecArray[i] << std::endl;

or

for(std::vector<int>::iterator it=vecArray.begin();it!=vecArray.end();++it)
    std::cout << "ArrayValue: " << *it << std::endl;

The operator

sizeof

does NOT give you the length of an array. Instead, it returns an unsigned integer representing the number of bytes of the type you give as the argument. For example,

std::cout << sizeof(unsigned long long) << std::endl;

prints

8

on my machine, because the type unsigned long long consista of 64-bits, or 64/8 = 8 bytes.

In C++11 we now have range-based for-loops:

Examples:

for(int i : vecArray)
    std::cout << "i = " << i << std::endl;

for(const int& i : vecArray)
    std::cout << "i = " << i << std::endl;

In the first example the values are copied from vecArray into the variable i. In the second example you are instead working with const references to the values in vecArray (which could be useful for objects which are expensive to copy).

In fact, the range-based for loops in C++11 are available for all types on which you can call begin() and end() (ie those which you can iterate through).