迭代const char *数组

Question

I have an really simple example of array of const char's and one function supposed to print them out (iterate through the chosen one). Contrary all my expectations, it's iterating through all of them and not only the one that was passed as argument.

#include <iostream>

const char* oranges[] = {
    "ORANGE",
    "RED ORANGE"
};

const char* apples[] = {
    "APPLE"
};

const char* lemons[] = {
    "LEMON"
};

void printFruit(const char** fruit){
    int i =0;
    while (fruit[i] != '\0'){
        std::cout << "---------------------\n";
        std::cout << fruit[i] << "\n";
        i++;
    }
}

int main (int argc, const char * argv[])
{
   printFruit(oranges); 
   return 0;
}

The result i would expect is that the function printFruit with oranges given as argument will print ORANGE and RED ORANGE, meanwhile i get printed ALL of the fruits defined (from other arrays), like this:

---------------------
ORANGE
---------------------
RED ORANGE
---------------------
APPLE
---------------------
LEMON

Sorry for my ignorance but why is this happening ?

Edit: I followed this question: defining and iterating through array of strings in c that is similar to mine.

Answer 1

You are having UB here. Your condition

while (fruit[i] != '\0')

will never be met because there are no elements that are equal to \\0 .

All the arrays are placed exactly one after another in the memory. Your i keeps increasing forever. On i = 1 you are on the first string in oranges . On i = 2 you are on the second element.

After that, i becomes 3. Since right after oranges , in your mamory lies the apples array, your pointer starts pointing to it and the app prints APPLE . On i = 4 the pointer is on the lemons array and the app prints LEMONS . After that you effectively go out of your own memory which for me results in a crash.

To fix that you need to explicitly add an empty element into each of the arrays, eg

const char* oranges[] = {
    "ORANGE",
    "RED ORANGE",
    0
};

Answer 2

You are checking that fruit[i] != '\\0' . That is wrong because fruit[i] is a char * , not a char. Furthermore, your vectors aren't terminated. You probably wanted to check whether fruit[i] != 0 , or *fruit[i] != '\\0' . In the first case, you need to terminate the vectors like this:

const char* oranges[] = {
    "ORANGE",
    "RED ORANGE",
    0  // or NULL
};

In the second:

const char* oranges[] = {
    "ORANGE",
    "RED ORANGE",
    ""
};

Answer 3

IMHO, you'd be better off knowing exactly how many elements that you're dealing with. The bad news is that a simple array of character pointers won't tell you (it's not a std::vector ) so you won't be able to discover it in your printFruit function.

The good news, however, is that it is available at compile time so you don't have to worry about the overhead of finding it out. The following shows what I mean:

void printFruit(const char** fruit, int fruitSize){
    int i =0;
    while (i < fruitSize){
        std::cout << "---------------------\n";
        std::cout << fruit[i] << "\n";
        i++;
    }
}

int main (int argc, const char * argv[])
{
   // The second parameter can be worked out by the compiler.
   printFruit(oranges, sizeof(oranges)/sizeof(const char*) ); 
   return 0;
}

Since you're using C++ though, I'd strongly recommend that you use one of the standard collection types such as vector as they're much safer when it comes to bounds checking, memory allocation etc.

Answer 4

const char* oranges[] = {
    "ORANGE",
    "RED ORANGE"
};

const char* apples[] = {
    "APPLE"
};

const char* lemons[] = {
    "LEMON"
};

in memory will looks somthing like

"ORANGE""RED ORANGE""APPLE""LEMON"

while (fruit[i] != '\0'){
    std::cout << "---------------------\n";
    std::cout << fruit[i] << "\n";
    i++;
}

will end when you reach the end of "big array" which is "LEMON"

to make your code working you need memory to looks like "ORANGE""RED ORANGE"0"APPLE"0"LEMON"0 so

   const char* oranges[] = {
        "ORANGE",
        "RED ORANGE",
    0
    };

    const char* apples[] = {
        "APPLE",
    0
    };

    const char* lemons[] = {
        "LEMON"
     ,0
    };

Answer 5

const char* oranges[] = {
    "ORANGE",
    "RED ORANGE",
    "\0"
};

Your arrays are located in memory one after another so it continues printing others because the while condition is false