user9418 user9418 - 3 months ago 27
C++ Question

What is wrong with strncpy_s() here?

I am reading my a textbook and trying to solve the question given to readers.

The fallowing code is function definition from my source file of my answer.

I want to copy content of character strings to another ones.

I chose functions strncpy_s().

But it does not work.

Microsoft Visual Studio says Debug Assertion Failed!

I have no idea how to fix it.


cow.h


// class declarations

#include <iostream>
#ifndef COW_H_
#define COW_H_

class Cow {
char name[20];
char * hobby;
double weight;
public:
Cow();
Cow(const char * nm, const char * ho, double wt);
Cow(const Cow & c);
~Cow();
Cow & operator=(const Cow & c);
void ShowCow() const; // display all cow data
};
#endif



cow.cpp


// class methods

Cow::Cow(const char * nm, const char * ho, double wt)
{
int len = std::strlen(nm);
strncpy_s(name, len, nm, len);
name[19] = '\0';

len = std::strlen(ho);
hobby = new char[len + 1];
strncpy_s(hobby, len, ho, len);
hobby[len] = '\0';

weight = wt;
}

Cow::Cow()
{
strncpy_s(name, 19, "no name", 19);
name[19] = '\0';

int len = std::strlen("no hobby");
hobby = new char[len + 1];
strncpy_s(hobby, len, "no hobby", len);
hobby[len] = '\0';

weight = 0.0;
}

Cow::Cow(const Cow & c)
{
int len = std::strlen(c.name);
strncpy_s(name, len, c.name, len);
name[19] = '\0';

len = std::strlen(c.hobby);
hobby = new char[len + 1];
strncpy_s(hobby, len, c.hobby, len);
hobby[len] = '\0';

weight = c.weight;
}

Cow::~Cow()
{
delete [] hobby;
}

Cow & Cow::operator=(const Cow & c)
{
if (this == &c)
return * this;

delete [] hobby;

int len = std::strlen(c.name);
strncpy_s(name, len, c.name, len);
name[19] = '\0';

len = std::strlen(c.hobby);
hobby = new char[len + 1];
strncpy_s(hobby, len, c.hobby, len);
hobby[len] = '\0';

weight = c.weight;
return * this;
}

void Cow::ShowCow() const
{
cout << name << ", " << hobby << ", " << weight << endl;
}



usecow.cpp


#include <iostream>
#include "cow.h"


int main()
{
Cow Japan;
Japan.ShowCow();

Cow America("Aspen", "Swim", 307.45);
America.ShowCow();

return 0;
}

Answer

From strncpy_s documentation:

These functions try to copy the first D characters of strSource to strDest, where D is the lesser of count and the length of strSource. If those D characters will fit within strDest (whose size is given as numberOfElements) and still leave room for a null terminator, then those characters are copied and a terminating null is appended; otherwise, strDest[0] is set to the null character and the invalid parameter handler is invoked, as described in Parameter Validation.

Lets consider your code:

int len = std::strlen("no hobby");
hobby = new char[len + 1];
strncpy_s(hobby, len, "no hobby", len);

Second argument of strcpy_s is a size of buffer in characters. Fourth - number of characters copied. Since you are passing the same len variable, strcpy_s detects, that buffer has insufficient size (because there should be a space for the trailing \0) and invokes invalid parameters handler. This works correctly:

int len = std::strlen("no hobby");
hobby = new char[len + 1];
strncpy_s(hobby, len+1, "no hobby", len);

Check other places where you are using strncpy_s for this error. Also it is a good idea to actually read text in the debug assertion windows. In this case error source is pretty straightforward:

debug assertion failed

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