Question

I am trying to use the new C++11 range based for loops. Here’s my program:

#include <iostream>
#include <string>
#include <sstream>
#include <fstream>

using namespace std;

ofstream logger("log.txt");
void log(string message)
{
    logger << message << std::endl;
    logger.flush();
}

int main( int argc, char* args[] )
{
    log("hello world");
    cout << "hello world\n";

    log("declare sort me");
    int sortMe[10];

    log("loop sortMe");
    for(int i : sortMe) {
        log("in loop " + i);
        sortMe[i] = i + 1;
    }
}

I’m using clang++ to compile. It compiles with the warning:

clang++ -o mycpp mycpp.cpp
mycpp.cpp:24:12: warning: range-based for loop is a C++11 extension
      [-Wc++11-extensions]
        for(int i : sortMe) {
                  ^
1 warning generated.

When it runs, I get this output:

hello world
Segmentation fault (core dumped)

According to the log.txt file, the program gets to the for loop, but it never enters the for loop. What am I missing?

Answer 1

This loop:

for(int i : sortMe) {
    log("in loop " + i);
    sortMe[i] = i + 1;
}

Loops and returns the values stored in the sortMe array, not the indices of the sortMe array. As a result, the lookup sortMe[i] will jump to a totally random index of the array (probably way, way out of bounds), causing the segfault.

If you want to set each element equal to its position, just use a normal for loop:

for (int i = 0; i < 10; i++) {
    sortMe[i] = i + 1;
}

Also, as @hmjd noted, the call to log will not work correctly, because you are doing pointer arithmetic on a string, not doing a string concatenation.

Hope this helps!