c++: passing arrays by reference

Question

I am trying to define a function prototype which takes an array of char of different lengths. I understand that I must pass the array by reference to avoid the array decaying to a pointer to its first element. So I've been working on this simple example to get my understanding correct.

 #include <stdio.h> // size_t

 //template to accept different length arrays
 template<size_t len>
 //pass array of char's by reference
 void func(const char (&str)[len])
 {
     //check to see if the array was passed correctly
     printf(str);
     printf("\n");
     //check to see if the length of the array is known
     printf("len: %lu",len);
 }
 int main(){
     //create a test array of chars
     const char str[] = "test12345";
     //pass by reference
     func(&str);
     return 0;
 }

This gives me the compiler errors:

main.cpp: In function ‘int main()’:
main.cpp:19:14: error: no matching function for call to ‘func(const char (*)[10])’
     func(&str);
              ^
main.cpp:6:6: note: candidate: template<long unsigned int len> void func(const char (&)[len])
 void func(const char (&str)[len])
      ^~~~
main.cpp:6:6: note:   template argument deduction/substitution failed:
main.cpp:19:14: note:   mismatched types ‘const char [len]’ and ‘const char (*)[10]’
     func(&str);

I thought that the function signature func(const char (&str)[len]) indicates a pointer to a char array of length len, which is what I am passing by func(&str).

I tried func(str), which I would expect to be wrong, since I am passing the value str, instead of its reference. However, this actually works and I dont understand why.

What is going on here? What does it actually mean to pass by reference?

Answer 1

Your function is declared correctly, but you are not passing the array to it correctly.

func(*str); first decays the array to a pointer to the 1st element, and then deferences that pointer, thus passing just the 1st character to func(). But there is no func(char) function defined, so this is an error.

func(&str); takes the address of the array, thus passing a pointer to the array, not a reference to it. But there is no func(char(*)[len]) function defined, so this is also an error.

To pass str by reference, you need to simply pass str as-is without * or &:

func(str);

This is no different than passing a reference to a variable of any other type, eg:

void func(int &value);

int i;
func(i);

---

On a side note: printf(str); is dangerous, since you don't know if str contains any % characters in it. A safer call would be either:

printf("%s", str);

Or:

puts(str);

But those only work if str is null-terminated (which it is in your case). Even safer would be:

printf("%.s", (int)len, str);

Which doesn't require a null terminator.