Case Study on cross-border problem of C ++ character array

Instructor he:

Instructor, m_szDepartment = new char [strlen (department) + 1]; why do we need to add 1? After removing + 1 in the test, the effect is the same. I don't quite understand it ...... Please advise ......
Student, Yang tengfei

My answer:
You need to assign blocks to '\ 0. I saved my own information using another person's website (cross-border part). The running results will be different if someone else wants to use it. This is the most dangerous problem.

He continued to ask:
However, when defining character arrays, such as a [4], you can enter five characters. Is the '\ 0' position of this array also occupied by others?

I am excited about the problem of this clever classmate. I should not just read the program but ask questions. If you can raise your own questions, you will be able to learn well. The teacher answers the question, asks a new question, answers it by himself, or asks the teacher again. This is the communication. I am proud to have raised a good question for the students. My answer is:

The question you asked is not average. Please refer to my new blog post to discuss this question.

 

The following is my answer to this question. Try a program to solve the problem. The program is:

[Cpp]
# Include <iostream>
Using namespace std;
Int main ()
{
Char a [4];
Cin>;
Cout <a <endl;
Return 0;
}

# Include <iostream>
Using namespace std;
Int main ()
{
Char a [4];
Cin>;
Cout <a <endl;
Return 0;
} Dear reader, please do not just read this article to open a familiar programming environment and run it while reading it. What do you find?

Enter abcd and press Enter. The output is abcd. Cout <a outputs character arrays as strings. Obviously, abcd is full of its own location a [0] to a [3] and can be output as expected, actually, the occupied memory a [4] unit is occupied. Of course, the face is '\ 0' at exactly a [4 '. If "hot" do not have to be surprised.

Run again and enter abcde. The result I run is that in VC ++ 6.0, the abcde is output, and a memory out-of-bounds error message is displayed. Under codeBlocks, output abcde and there is no prompt.
Please think about how dangerous this is.
A program is provided below:

[Cpp]
# Include <iostream>
Using namespace std;
Int main ()
{
Char a [4], B [4];
Cin>;
Cin> B;
Cout <a <endl;
Cout <B <endl;
Return 0;
}

# Include <iostream>
Using namespace std;
Int main ()
{
Char a [4], B [4];
Cin>;
Cin> B;
Cout <a <endl;
Cout <B <endl;
Return 0;
} The running task is handed over to the reader. If you enter 3 characters, 4 characters, and 5 characters, you can try it on multiple platforms. It is strongly recommended to track data storage in the memory by means of one-step execution.

The following are three characters (abc and hij) for Array a and array B. The result is displayed in one step:

　　
 

The following figure shows the result after entering 5 characters (abcef and hijkl) for Array a and array B:

　　

From this we can see that in VC ++ 6.0, the address of the first defined array a is greater than the address of the second defined array B. Originally, abcde was entered for Array a, occupying others' territory, then, input hijkl for B, which encroaches on a's territory. B [4] means that a [0] is l, B [5] is a [1], store '\ 0 '!

It is the result of debugging with the same input in codeBlocks. The result is the same:

　　

Next, let's change the order of input a and input B to another program:

[Cpp]
# Include <iostream>
Using namespace std;
Int main ()
{
Char a [4], B [4];
Cin> B;
Cin>;
Cout <a <endl;
Cout <B <endl;
Return 0;
}

# Include <iostream>
Using namespace std;
Int main ()
{
Char a [4], B [4];
Cin> B;
Cin>;
Cout <a <endl;
Cout <B <endl;
Return 0;
} What is the running result? Let's talk to you. Don't forget. Use the debugging tool to relieve your doubts.