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I am from Python background and recently learning C++. I was learning a C/C++ function called memset and following the online example from website https://www.geeksforgeeks.org/memset-in-cpp/ where I got some compilation errors:

/**
 * @author : Bhishan Poudel
 * @file : a02_memset_geeks.cpp
 * @created : Wednesday Jun 05, 2019 11:07:03 EDT
 * 
 * Ref: 
 */

#include <iostream>
#include <vector>
#include <cstring>

using namespace std;

int main(int argc, char *argv[])
 char str[] = "geeksforgeeks";

 //memset(str, "t", sizeof(str));
 memset(str, 't', sizeof(str));

 cout << str << endl;

 return 0;

Error when using single quotes 't'

This prints extra characters.

tttttttttttttt!R@`

Error when using "t" with double quotes

$ g++ -std=c++11 a02_memset_geeks.cpp 
a02_memset_geeks.cpp:17:5: error: no matching function for call to 'memset'
 memset(str, "t", sizeof(str));
 ^~~~~~
/usr/include/string.h:74:7: note: candidate function not viable: no known
 conversion from 'const char [2]' to 'int' for 2nd argument
void *memset(void *, int, size_t);
 ^
1 error generated.

How to use the memset in C++ ?

Further Study

Excellent tutorial with shortcomings of memset is given here:
https://web.archive.org/web/20170702122030/https:/augias.org/paercebal/tech_doc/doc.en/cp.memset_is_evil.html

This declaration

char str[] = "geeksforgeeks";

declares a character array that contains a string that is a sequence of characters including the terminating zero symbol ''.

You can imagine the declaration the following equivalent way

char str[] = 
 
 'g', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'g', 'e', 'e', 'k', 's', ''
;

This call of the function memset

memset(str, 't', sizeof(str));

overrides all characters of the array including the terminating zero.

So the next statement

cout << str << endl;

results in undefined behaviour because it outpuuts characters until the terminating zero is encountered.

You could write instead

#include <iostream>
#include <cstring>

int main()

 char str[] = "geeksforgeeks";

 std::memset( str, 't', sizeof( str ) - 1 );

 std::cout << str << 'n';

Or the following way

#include <iostream>
#include <cstring>

int main()

 char str[] = "geeksforgeeks";

 std::memset( str, 't', std::strlen( str ) );

 std::cout << str << 'n';

That is keeping the terminating zero unchanged in the array.

If you want to override all characters of the array including the terminating zero, then you should substitute this statement

std::cout << str << 'n';

for this statement

std::cout.write( str, sizeof( str ) ) << 'n';

as it is shown in the program below because the array now does not contain a string.

#include <iostream>
#include <cstring>

int main()

 char str[] = "geeksforgeeks";

 std::memset( str, 't', sizeof( str ) );

 std::cout.write( str, sizeof( str ) ) << 'n';

As for this call

memset(str, "t", sizeof(str));

then the type of the second argument (that is the type const char *) does not correspond to the type of the second function parameter that has the type int. See the declaration of the function

void * memset ( void * ptr, int value, size_t num );

Thus the compiler issues an error message.

Error when using single quotes 't' This prints extra characters.

That's because you overwrote the null terminator.

The terminator is part of the array's size (an array is not magic), though it's not part of the logical string size.

So, I think you meant:

memset(str, 't', strlen(str));
// ^^^^^^

Error when using "t" with double quotes

Completely different thing. You told the computer to set every character in the string, to a string. Doesn't make sense; won't compile.

How to use memset in C++?

Don't.

Either use the type-safe std::fill, in combination with std::begin and std::end:

std::fill(std::begin(str), std::end(str)-1, 't');

Or just a std::string to begin with. 😊

I was learning the fuction memset in C++ from https://www.geeksforgeeks.org/memset-in-cpp/ where the example is given as below

Don't attempt to learn C++ from random websites. Get yourself a good book instead.

This is the correct syntax for memset...

void* memset( void* dest, int ch, std::size_t count );

Converts the value ch to unsigned char and copies it into each of the first count characters of the object pointed to by dest. If the object is a potentially-overlapping subobject or is not TriviallyCopyable (e.g., scalar, C-compatible struct, or an array of trivially copyable type), the behaviour is undefined. If count is greater than the size of the object pointed to by dest, the behaviour is undefined.

For the first syntax memset(str, 't', sizeof(str));. The compiler complained because of extra size. It prints 18 times tttttttttttttt!R@. I suggest try with sizeof(str) -1 for char array.

For Second syntax memset(str, "t", sizeof(str)); you are providing the second parameter is a string. This is the reason compiler complains error: invalid conversion from ‘const char*’ to ‘int’