strlen() not working with string variable

Question

#include <iostream>
#include <string>
#include <cstring>


using namespace std;

int main()
{
    string b="hello";

    cout<<b;

    int c = strlen(b);

    cout << "Hello world!" <<c<< endl;
    return 0;
}

When I try to run this I get the error below

||=== Build: Debug in strlen (compiler: GNU GCC Compiler) ===|
C:\Users\Waters\Desktop\hellow world\strlen\main.cpp||In function 'int main()':|
C:\Users\Waters\Desktop\hellow world\strlen\main.cpp|14|error: cannot convert 'std::string {aka std::basic_string<char>}' to 'const char*' for argument '1' to 'size_t strlen(const char*)'|
||=== Build failed: 1 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

@WatersSharma: because `strlen` takes a `char *` argument, not an `std::string`, which is totally meaningless to it. `std::string::c_str()` is a function returning a C string (`const char *`). — Nick Matteo, May 28 '16 at 01:23
@WatersSharma: if you bothered to read the error message, it says right there: `cannot convert 'std::string {aka std::basic_string}' to 'const char*' for argument '1' to 'size_t strlen(const char*)'` — dgsomerton, May 28 '16 at 01:24
Whatever source you're learning from, throw it out and get a decent book. — molbdnilo, May 28 '16 at 01:28

score 9 · Accepted Answer · answered May 28 '16 at 01:25

strlen is a function from C and works with C strings (char *).

For proper C++ std::strings use the .length() or .size() member functions.

In fact most standard headers that start with 'c' include functions that C++ inherited from C. In your case you most likely don't have any reason to use <cstring> if you're already working with C++ strings.

Peregrin · Answer 2 · 2016-05-28T20:41:09.287

0

your case will work if you just changed

string b="hello";

to

char* b = "hello";

note that C strings always (should) have null character '\0' that determines that end of the string. It's explained briefly for example here

edited May 28 '16 at 20:41

answered May 28 '16 at 20:33

Peregrin

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You surely meant, C strings must be terminated by `'\0'` not by `'\n'`. – cwschmidt May 28 '16 at 20:40
@cwschmidt aha, you are damn right :D I should probably get some sleep – Peregrin May 28 '16 at 20:42
also `const char *b` instead of `char *b`. – coyotte508 May 28 '16 at 20:45
@cwschmidt well, in the example string isn't const – Peregrin May 30 '16 at 08:40
@Peregrin What do you refer to? It's irrelevant if it's `const` or not, C strings have to be terminated by `'\0'` – cwschmidt May 30 '16 at 11:22

strlen() not working with string variable

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