问题
For the purpose of education, I'm writing a small pair of functions which open a file as a binary file, read each character one at a time, change the value of said character and write to a new, encrypted file. (Note, this isn't true encryption per se.)
For some reason, I'm getting this error message in my debugger at the while-loop condition:
Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)
I suspect the issue is with understanding pointer. Here is my program in it's entirety. Thanks ahead of time.
// This program reads the contents of a file, encrypts it, and write the contents into a separate file.
#include <iostream>
#include <fstream>
#include <cctype>
#include <cstring>
#include <cmath>
using namespace std;
// Global Constants
const int POSITIVE_INT_LIMIT = 10;
const int NEGATIVE_INT_LIMIT = -10;
const int SLOPE_NEGATIVE = -1;
const int SLOPE_POSITIVE = 1;
const int STEP = 1;
// Function Prototypes
void encrypt(fstream *unEncryptedFile);
fstream *fileOpener();
int main() {
encrypt(fileOpener());
return 0;
}
/****************************************************
* fileOpener() *
* Opens file with error checking and displays *
* message if error, then exiting gracefully. *
****************************************************/
fstream *fileOpener(){
fstream inFile; // Create fstream object
inFile.open("crypty.txt", ios::in | ios::binary); // Open file
if (!inFile) {
cout << "Error opening file." << endl;
return NULL;
} else {
fstream *filePtr; // create a pointer to the file.
filePtr = &inFile; // Pass the address of inFile to the pointer.
return filePtr;
}
}
/***************************************************
* encrypt() *
* Encrypts a file with a simlpe algorithm. *
***************************************************/
void encrypt(fstream &unEncryptedFile){
char ch, cy;
char *chPtr = &cy; // Initialize and assign char pointer.
fstream cryptFile;
cryptFile.open("decrypy.txt", ios::out | ios::binary);
int amplitude = 0, slope = SLOPE_NEGATIVE; //Set initial amplitude and slope.
int streamSize = 1;
/* Encryption pattern inside while-loop.
limit> 10 *
9 * *
8 * *
7 * *
6 * *
5 * *
4 * *
3 * *
2 * *
1 * *
start> 0*2345678901234567890*2345678901234567890* -- < one hertz (cycle)
-1 * *
-2 * * (Number line: each integer represents a single while loop cycle.)
-3 * *
-4 * *
-5 * *
-6 * *
-7 * *
-8 * *
-9 * *
limit> -10 *
*/
/*************************************************************
The pattern above depictes a single character
being read, and then the value of amplitude is added to it.
*************************************************************/
while (!unEncryptedFile.fail()) { // <--Code reports bug at this statement.
ch = unEncryptedFile.get(); // Get the next character in the file.
if (amplitude > NEGATIVE_INT_LIMIT && slope == SLOPE_NEGATIVE) {
amplitude -= STEP;
cy = ch + amplitude;
cryptFile.write(chPtr, streamSize); //Adjusted character value, cy, is written to file.
} else if (amplitude <= NEGATIVE_INT_LIMIT){
slope = SLOPE_POSITIVE;
amplitude = NEGATIVE_INT_LIMIT;
cy = ch + amplitude;
cryptFile.write(chPtr, streamSize); //Adjusted character value, cy, is written to file.
} else if (amplitude < POSITIVE_INT_LIMIT && SLOPE_POSITIVE){
amplitude += STEP;
cy = ch + amplitude;
cryptFile.write(chPtr, streamSize); //Adjusted character value, cy, is written to file.
} else if (amplitude >= POSITIVE_INT_LIMIT){
slope = SLOPE_NEGATIVE;
amplitude = POSITIVE_INT_LIMIT;
cy = ch + amplitude;
cryptFile.write(chPtr, streamSize); //Adjusted character value, cy, is written to file.
}
}
//Files are closed.
unEncryptedFile.close();
cryptFile.close();
}
回答1:
I suspect the issue is with understanding pointer.
Probably, almost you right, but with understanding of pointers to local variables.
When you define local variables like this:
fstream inFile; // Create fstream object
inFile is allocated on the stack.
With following:
filePtr = &inFile
... you'll get pointer to some place on the stack in filePtr.
And when you exit that function:
return filePtr;
... stack portion, associated with function call will be returned to heap and filePtr will point to unused memory address, which will be overriden by next called function/executed code.
So, why not just return inFile itself?
来源：https://stackoverflow.com/questions/34034426/issue-with-encryption-function-not-linking-to-main