解析C++中的for循环以及基于范围的for语句使用

for ( init-expression ; cond-expression ; loop-expression ) 
  statement;

#include <iostream>
using namespace std;

int main() {
  // The counter variable can be declared in the init-expression.
  for (int i = 0; i < 2; i++ ){ 
    cout << i;
  }
  // Output: 01
  // The counter variable can be declared outside the for loop.
  int i;
  for (i = 0; i < 2; i++){
    cout << i;
  }
  // Output: 01
  // These for loops are the equivalent of a while loop.
  i = 0;
  while (i < 2){
    cout << i++;
  }
}
  // Output: 012
init-expression 和 loop-expression 可以包含以逗号分隔的多个语句。例如：


#include <iostream>
using namespace std;

int main(){
  int i, j;
  for ( i = 5, j = 10 ; i + j < 20; i++, j++ ) {
    cout << "i + j = " << (i + j) << '\n';
  }
}
  // Output:
  i + j = 15
  i + j = 17
  i + j = 19

#include <iostream>
using namespace std;

int main(){
for (int i = 10; i > 0; i--) {
    cout << i << ' ';
  }
  // Output: 10 9 8 7 6 5 4 3 2 1
  for (int i = 10; i < 20; i = i+2) {
    cout << i << ' ';
  }
  // Output: 10 12 14 16 18

#include <iostream>
using namespace std;

int main()
{
  int i;
  for( i = 0; i < 5; cout << i << '\n', i++){
    ;
  }
}

for (int i = 0 ; i < 5 ; i++) {
  // do something
}
// i is now out of scope under /Za or /Zc:forScope

// for_statement5.cpp
int main(){
  int i = 0;  // hidden by var with same name declared in for loop
  for ( int i = 0 ; i < 3; i++ ) {}

  for ( int i = 0 ; i < 3; i++ ) {}
}

  for ( for-range-declaration : expression )
statement 

// range-based-for.cpp
// compile by using: cl /EHsc /nologo /W4
#include <iostream>
#include <vector>
using namespace std;

int main() 
{
  // Basic 10-element integer array.
  int x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

  // Range-based for loop to iterate through the array.
  for( int y : x ) { // Access by value using a copy declared as a specific type. 
            // Not preferred.
    cout << y << " ";
  }
  cout << endl;

  // The auto keyword causes type inference to be used. Preferred.

  for( auto y : x ) { // Copy of 'x', almost always undesirable
    cout << y << " ";
  }
  cout << endl;

  for( auto &y : x ) { // Type inference by reference.
    // Observes and/or modifies in-place. Preferred when modify is needed.
    cout << y << " ";
  }
  cout << endl;

  for( const auto &y : x ) { // Type inference by reference.
    // Observes in-place. Preferred when no modify is needed.
    cout << y << " ";
  }
  cout << endl;
  cout << "end of integer array test" << endl;
  cout << endl;

  // Create a vector object that contains 10 elements.
  vector<double> v;
  for (int i = 0; i < 10; ++i) {
    v.push_back(i + 0.14159);
  }

  // Range-based for loop to iterate through the vector, observing in-place.
  for( const auto &j : v ) {
    cout << j << " ";
  }
  cout << endl;
  cout << "end of vector test" << endl;
}

1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
end of integer array test
0.14159 1.14159 2.14159 3.14159 4.14159 5.14159 6.14159 7.14159 8.14159 9.14159
end of vector test