4

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

jleahy
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KItis
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4 Answers4

13

You are trying to use strlen function, which is declared in string.h (or, as a member of namespace std in cstring). So, in order to use strlen you should include one of those two headers.

The #include <string> variant does not work simply because string is a completely unrelated C++-specific header file which has absolutely nothing to do with C standard library string functions. What made you expect that it will work?

AnT stands with Russia
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7

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.

j_random_hacker
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4
#include<string>

this include is for c++ std::string, not c string (link: http://www.cppreference.com/wiki/string/start)

and those: 

#include<cstring>

and 

#include<string.h>

are for c strings (link: http://www.cplusplus.com/reference/clibrary/cstring/)

matekm
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0

use string.length() instead of strlen()

jeevanreddymandali
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