Substracting a string from an int

Question

I following code illustrates the use of c_str function

#include <iostream>
#include <string.h>

using namespace std;

int main() {
    std::string str("Hello world!");
    int pos1 = str.find_first_of('w');
    cout<< "pos1: "<< pos1 <<endl;
    int pos2 = strchr(str.c_str(), 'w') - str.c_str();   //*
   //int pos2 = strchr(str.c_str(), 'w')
    cout<< "pos2: "<< pos2 <<endl;
    cout<< "str.c_str(): "<< str.c_str() <<endl;
    if (pos1 == pos2) {
        printf("Both ways give the same result.\n");
    }
}

The output is

pos1: 6
pos2: 6
str.c_str(): Hello world!
Both ways give the same result.

I don't get the str.c_str() role in line * . I am substracting a string from an int, what is the meaning of that? When I erase it, that is when I comment line * , and uncomment the following line I get an error: invalid conversion from 'char*' to 'int'. How come there is not error in the original code?

No, you're subtracting a pointer from a pointer, which gives you the distance between the two addresses. Why do you think `strchr` returns an `int`? — molbdnilo, Mar 17 '21 at 08:58
@molbdnilo, I see, then why printing str.c_str() outputs the full string instead of the pointer? — some_math_guy, Mar 17 '21 at 09:02
`operator<<` has an overload for char pointers that expects a null-terminated string and pritns that. Thats why you can do eg `std::cout << "Hello world";` — 463035818_is_not_an_ai, Mar 17 '21 at 09:03
@largest_prime_is_463035818 In order to see the pointer value I was trying " cout<< "str.c_str(): "<< &(str.c_str()) < — some_math_guy, Mar 17 '21 at 09:08
to see the value of the pointer you can cast it to `void*`, eg `std::cout << static_cast(str.c_str());` the overload for `void*` prints the value of the pointer — 463035818_is_not_an_ai, Mar 17 '21 at 09:10
https://stackoverflow.com/questions/9377407/displaying-the-address-of-a-string — 463035818_is_not_an_ai, Mar 17 '21 at 09:10
@largest_prime_is_463035818 How come the address operator doesn't give me the value as I thought? — some_math_guy, Mar 17 '21 at 09:12
because an lvalue is required as unary & operand ;). https://stackoverflow.com/questions/22788026/error-lvalue-required-as-unary-operand — 463035818_is_not_an_ai, Mar 17 '21 at 09:13
well the question i linked is about taking & of the result of a cast, but answer is almost same for a temporary returned from a function. — 463035818_is_not_an_ai, Mar 17 '21 at 09:14
@largest_prime_is_463035818 So If I understood well , str.c_str() yields a temporary that is not stored anywhere, so you cannot assign to it the returned value from the & operator? — some_math_guy, Mar 17 '21 at 09:22

SchweizS · Accepted Answer · 2021-03-17T09:07:45.833

2

.c_str() returns the address to the start of the string. strchr the address to the first occurence of a specific character inside the given string or a nullptr, if the character is not found.

If you subtract one address from another, you get the distance of the to pointers, which is the offset of the character inside the string in this case.

The find_* functions of the string class all return the offset or std::string::npos, if the character is not found.

Reference:

edited Mar 17 '21 at 09:07

answered Mar 17 '21 at 09:05

SchweizS

44
4

note that subtracting one pointer from another is only defined for pointers into the same array – 463035818_is_not_an_ai Mar 17 '21 at 09:06

score 0 · Answer 2 · answered Mar 17 '21 at 09:42

0

First,strchr returns a pointer.Pointrer types require a forced cast. Second,The data type of the pointer is related to the number of CPU bits.So you should use unsigned long type.

answered Mar 17 '21 at 09:42

DFChelsea

1

Substracting a string from an int

2 Answers2