C ++通过带有辅助函数的引用传递

Question

void BTreeTest::testOneBTreeLeafNode() {

BTreeLeafNode *test = new BTreeLeafNode();

BTreeData *info1 = new BTreeData;
BTreeData *info2 = new BTreeData;
BTreeData *info3 = new BTreeData;
BTreeData *info4 = new BTreeData;
BTreeData *info5 = new BTreeData;


setInfo(&*info1, (char *)"Lester", (char *)"24", (char *)"student", 6, 2, 7);
setInfo(&*info2, (char *)"David", (char *)"20", (char *)"student", 5, 2, 7);
setInfo(&*info3, (char *)"June", (char *)"4", (char *)"toddler", 4, 1, 7);
setInfo(&*info4, (char *)"Lisa", (char *)"18", (char *)"tutor", 4, 2, 5);
setInfo(&*info5, (char *)"Savannah", (char *)"22", (char *)"barista", 8, 2, 7);

correctData(&*info1, (char *)"Lester", (char *)"24", (char *)"student");
correctData(&*info2, (char *)"David", (char *)"20", (char *)"student");
correctData(&*info3, (char *)"June", (char *)"4", (char *)"toddler");
correctData(&*info4, (char *)"Lisa", (char *)"18", (char *)"tutor");
correctData(&*info5, (char *)"Savannah", (char *)"22", (char *)"barista");


delete info1;
delete info2;
delete info3;
delete info4;
delete info5;

delete test;

}

This is a test I wrote for a program I'm trying to build. But, in order for me to have a cleaner more readable test I needed a helper function. That's what the setInfo is for. This is the code for that:

void setInfo(BTreeData *info, char *name, char *age, char *occupation, int  
   nameSize, int ageSize, int occSize) {

   strncpy(info->name, name, nameSize);
   strncpy(info->age, age, ageSize);
   strncpy(info->occupation, occupation, occSize);
}

The problem with this is I'm trying to set the value of info. And I'm trying to pass by reference so I can set the info. When I check through the xCode debugger it works correctly until the end of setInfo function. For some reason though, when my code goes back inside testOneBTreeLeafNode to continue on through the rest of the code, the values inside the infos change. It has extra unwanted garbage like the picture below. I have a poor understanding of passing by reference. Can someone enlighten me?

Notice how David's age and occupation should be 20 and student. It has 20 and student but also other unwanted garbage in it. Why is this happening?

In case it's necessary here are are the BTree structs:

//
//  BTree.hpp
//  CS130FinalProject
//
//  Created by Lester Dela Cruz on 2/19/16.
//  Copyright © 2016 Lester Dela Cruz. All rights reserved.
//

#ifndef BTree_hpp
#define BTree_hpp

#define M (5)
#define L (3)

#include <stdio.h>

struct BTreeData {
  char name[20];
  char age[3];
  char occupation[30];
};

struct BTreeLeafNode {
  BTreeData dataItems[L];
};

struct BTreeInternalNode {
  char keys[M-1][20];
  BTreeLeafNode *branch[M];
};

class BTree {

};

#endif /* BTree_hpp */

Answer 1

This is because you're not copying the terminating null character.

You are allocating a new instance of the BtreeData class on the heap. The initial copy of each instance contains random binary garbage. POD data allocated on the heap does not get initialized with zeroes.

Then you're using strncpy () to initialize the fields. To initialize the age member to "20", you're passing 2 for ageSize , so strncpy () is going to copy the '2' and the '0', but not the trailing \\0 that should be terminating C-style strings. So, your debugger does not see the trailing 0, then continues to print whatever random garbage it finds, because, as I said, the class instances were allocating from the heap and inherited whatever random data was previously, in the heap-allocated memory.

A few other bits of well meaning criticism, as long as I have your attention:

setInfo(&*info1, (char *)"Lester", (char *)"24", (char *)"student", 6, 2, 7);

First of all, the "&*" accomplishes absolutely nothing. It makes no difference whatsoever.

Secondly, the explicit (char *) cast is poorly advised. The reason that you have to do it is because string literals are const char s, and your setInfo () function takes char * as parameters.

Except that your setInfo () does not need to take char * parameters in the first place. Let's change setInfo () to take const char * as parameters, and get rid of these ugly, ugly casts.

This looks like a part of a larger application, so perhaps there's a reason why you're passing an explicit string length count, but there's no actual reason for that. Just let things take their natural course, use strcpy () instead of strncpy (), and forget about character counts.

Better yet, since we're talking C++ here, let's get rid of all your char arrays, and replace them with std::string s. Plain char arrays are so ...last century.

Do we have a deal?