getting the length of an array using strlen in g++ compiler

could someone explain why i am getting this error when i am compiling the source using following g++ compiler

#include <cstdio>
#include <string>

using namespace std;

int main()
{
    char source_language[50];


    scanf("%16s\n",source_language);

    int length = sizeof(source_language);
    int sizeofchar = strlen(source_language);
    printf("%d\n",sizeofchar);
}

this gives me following error

test.cpp: In function ‘int main()’:

test.cpp:31: error: ‘strlen’ was not declared in this scope

when i change the #include <string> into #include <string.h> or #include<cstring> , it works fine, i need to figure out what is the difference using #include<string> and #include<string.h> . really appreciate any help

Solution

C++ programmers normally have to deal with at least 2 flavours of string: raw C-style strings, usually declared as char *str; or char str[123];, which can be manipulated with strlen() etc.; and C++-style strings, which have the type std::string and are manipulated with member functions like string::length(). Unfortunately this leads to a bit of confusion.

In C, #include <string.h> declares strlen() et al.
In C++, you need #include <cstring> instead, which declares them in the std namespace, so you can either call these functions as std::strlen() etc. or you need to follow up with using namespace std;, in which case you can then just call them as strlen() etc. as usual.
C++ also has a totally separate header called <string>, which declares the C++ type std::string. This header has nothing to do with strlen(), so including it will not let you access strlen().

I don't know why Mehrdad Afshari deleted his answer, which I'm essentially repeating here.