potatoes and carrots no bones: C/C++ Array Size and Length

Sunday, December 2, 2012

C/C++ Array Size and Length

Introduction

A simple note of array size and length test

Code

array_size_and_length.cpp

#include <iostream>

using namespace std;

/** Test basic array operation,
 * the simple basic array, not std::array
 *
 */
class TestClass {
    public:
        int _a;
        double _b;
        TestClass ()
        {
            _a = 1;
            _b = 1.1;
        }
        TestClass (int a, double b) : _a(a), _b(b)
        {
        }
};
int getLength(int nums[]);
int getLength(TestClass vals[]);
int main() {
    // an int array contains 3 elements
    int nums[] = {1, 2, 5};
    // TestClass array contains 2 elements
    TestClass vals[] = {TestClass(), TestClass(2, 2.2)};
    // output: total bytes of nums[]:                   12
    cout << "total bytes of nums[]: \t\t\t" << sizeof(nums) << endl;
    // output: element size of nums:                    4
    cout << "element size of nums: \t\t\t" << sizeof(nums[0]) << endl;
    // output: number of element (length) of nums[]:    3
    // this will work since sizeof(nums) is correct here
    cout << "number of element (length) in nums[]: \t" << sizeof(nums)/sizeof(nums[0]) << endl;
    //
    // output: wrong length of nums[]:                  1 (wrong value)
    //
    // Note: this will not work since only the address of nums[0] (a pointer)
    //       will be passed to getLength(int nums[]) function
    cout << "wrong length of nums[]: \t\t" << getLength(nums) << " (wrong value)" << endl;
    cout << endl;

    // output: total bytes of vals[]:                   32
    cout << "total bytes of vals[]: \t\t\t" << sizeof(vals) << endl;
    // output: element size of vals:                    16
    cout << "element size of vals: \t\t\t" << sizeof(vals[0]) << endl;
    // output: number of element (length) of vals[]:    2
    // this will work since sizeof(vals) is correct here
    cout << "number of element (length) in vals[]: \t" << sizeof(vals)/sizeof(vals[0]) << endl;
    //
    // output: wrong length of vals[]:                  0 (wrong value)
    //
    // Note: this will not work since only the address of vals[0] (a pointer)
    //       will be passed to getLength(TestClass vals[]) function
    cout << "wrong length of vals[]: \t\t" << getLength(vals) << " (wrong value)" << endl;
    cout << endl;

    system("PAUSE");
    return 0;
}
int getLength(int nums[]) {
    // sizeof(nums) -> 4, (pointer's size)
    // sizeof(nums[0]) -> 4, (size of int)
    // return value is 1

    return sizeof(nums)/sizeof(nums[0]);
}
int getLength(TestClass vals[]) {
    // sizeof(vals) -> 4, (pointer's size)
    // sizeof(vals[0]) -> 16, (size of TestClass)
    // return value is 0

    return sizeof(vals)/sizeof(vals[0]);
}