[SIM-2362662]

I'm trying to find the length of an array of strings.
Let's say I have this :

Code

string my_str[] = {"a", "b", "c", "d", "e", "f"};

In order to get the length of my_str, I can do:

Code

int n = sizeof(my_str) / sizeof(my_str[0]);

And everything works fine. But if I try to do that in a function, it doesn't work. Like this:

Code

void myfct(string strArr[]) {
    int n = sizeof(strArr) / sizeof(strArr[0]);
    cout << n;
}

int main() {
    string my_str[] = {"1", "2", "E", "E", "1", "E"};
    myfct(my_str);
}

I get 0.

So I would like to know why this happens and how else I can do this.

[saltycracker]

Well look at the warning my compiler generates?

warning: ‘sizeof’ on array function parameter ‘strArr’ will return size of ‘std::__cxx11::string* {aka std::__cxx11::basic_string<char>*}’ [-Wsizeof-array-argument]
std::cout << sizeof(strArr) << std::endl;

Hint... It becomes a pointer when you pass it to the function. If memory serves me here.. You have to pass the array size to the function too.

[akali]

I'm trying to find the length of an array of strings.
Let's say I have this :

Code

string my_str[] = {"a", "b", "c", "d", "e", "f"};

In order to get the length of my_str, I can do:

Code

int n = sizeof(my_str) / sizeof(my_str[0]);

And everything works fine. But if I try to do that in a function, it doesn't work. Like this:

Code

void myfct(string strArr[]) {
int n = sizeof(strArr) / sizeof(strArr[0]);
cout << n;
}

int main() {
string my_str[] = {"1", "2", "E", "E", "1", "E"};
myfct(my_str);
}

I get 0.

So I would like to know why this happens and how else I can do this.

sizeof(array) is done at compile time. Otherwise it would give you pointer size. Whenever you're dealing with pointers to arrays, the size needs to be passed as an argument as well. They do not know their own size.

[SIM-2362662]

sizeof(array) is done at compile time. Otherwise it would give you pointer size. Whenever you're dealing with pointers to arrays, the size needs to be passed as an argument as well. They do not know their own size.

So you're saying there is no way for my function to know the length of the array without me passing it as an argument. Right?

[saltycracker]

So you're saying there is no way for my function to know the length of the array without me passing it as an argument. Right?

You have to pass the array size or have some sort of sentinel value to terminate the array... Like a C string.

[creki]

gay but works

[SIM-2362662]

gay but works

You don't necessarily get empty if you exceed the index, do you?
I.think you just get whatever is in the memory

[akali]

gay but works

I'm pretty sure the code you posted results in undefined behavior. Because at some point, you're trying to call a method of an object that may not even exist in memory. The compiler might just be doing some trickery there and it's working. But no you shouldn't do that. And the only reason string functions in general don't need a length is becasue they check for a null terminator.

---

You don't necessarily get empty if you exceed the index, do you?
I.think you just get whatever is in the memory

Exactly. His code doesn't make any sense.

[zNinja]

PHP Code


<?php 
template <class T,class A>
size_t array_size(A&array){
    return sizeof(array)/ sizeof(T);
}
iint main(int argc,char*argv[])
{
std::string test[] = { "a", "b", "c","defg" };
std::cout << array_size<std::string>(test) << std::endl;
reeturn 0;
} 


[dracx]

Well am not sure bout those C++ string, butnin C char array, you simply have to loop over the starting address then stop counting the first time you hit a null byte. This is not the case for wide char array tho, each wide char array is composed of two bytes and is terminated by two nill bytes too. Am on mobile so cant properly supply at least some pseudo-code. Google "custom strlen".

[zNinja]

Well am not sure bout those C++ string, butnin C char array, you simply have to loop over the starting address then stop counting the first time you hit a null byte. This is not the case for wide char array tho, each wide char array is composed of two bytes and is terminated by two nill bytes too. Am on mobile so cant properly supply at least some pseudo-code. Google "custom strlen".

Well, C string(char array) is a little bit tricky, if you have const char array, you can get size easy. For pointer you can not use "null byte" as orientation, since array can contain binary content which can have multiple null bytes.
You must track size of array trough code, Like when you reading from a file with C api, you must retrieve size of file then allocate container (char array) and then read.

[dracx]

Well, C string(char array) is a little bit tricky, if you have const char array, you can get size easy.  For pointer you can not use "null byte" as orientation, since array can contain binary content which can have multiple null bytes.
You must track size of array trough code, Like when you reading from a file with C api, you must retrieve size of file then allocate container (char array) and then read.

We we're talkin about string A.K.A. char array, not blob of data, no?