Programmers must read the summary of c ++ pen questions (1)

This articleProgrammerSummary of the written test process, for programmersC ++The pen questions are summarized. I hope to share it with you. Below are some common questions:

1. Find the return value of the following function (Microsoft)

 
 

  
  
{  
  
  
int countx = 0;  
  
  
while(x)  
  
  
{  
  
  
countx ++;  
  
  
x = x&(x-1);  
  
  
}  
  
  
return countx;  
  
  
} 
 
 

Assume that x = 9999. Answer: 8

Train of Thought: Convert x to a binary system and check the number of contained 1.

2. What is "reference "? What should I pay attention to when declaring and using "reference?

A: A reference is the "alias" (alias) of a target variable. operations on an application are identical to operations on a variable. When declaring a reference, remember to initialize it. After the reference declaration is complete, the target variable name has two names, that is, the original name and reference name of the target, and the reference name cannot be used as the alias of other variable names. Declaring a reference is not a new variable. It only indicates that the reference name is an alias of the target variable name. It is not a data type, so the reference itself does not occupy the storage unit, the system does not allocate storage units to references. You cannot create an array reference.

3. What are the features of using "Reference" as a function parameter?

(1) passing a reference to a function has the same effect as passing a pointer. In this case, the parameter of the called function becomes an actual parameter variable or an alias of the object in the original main function, therefore, the operations on the parameters in the called function are the operations on the corresponding target object (in the main function.

(2) using the reference parameter to pass the function does not generate a copy of the real parameter in the memory, it is directly to operate on the real parameter; while using the general variable to pass the function parameter, when a function call occurs, you need to allocate storage units to the parameters. The parameters are copies of the real variables. if the object is passed, the copy constructor will also be called. Therefore, when the data transmitted by a parameter is large, it is better to use reference than to transmit a parameter using a common variable.

(3) Although using pointers as function parameters can achieve the same effect as using references, in the called function, storage units must be allocated to the parameters, the "* pointer variable name" format must be used repeatedly for calculation, which can easily lead to errors and the program reading is poor. On the other hand, at the call point of the main function, the variable address must be used as the real parameter. References are easier to use and clearer.

4. When do I need to use "regular reference "?

If you want to use references to improve program efficiency and protect the data transmitted to the function from being changed in the function, you should use regular references. Common Reference declaration method: const type identifier & reference name = target variable name;

Example 1

 
 

  
  
IntA;
  
  
Const Int& Ra =;
  
  
Ra = 1;// Error 
  
  
A = 1;// Correct 
 
 

Example 2

 
 

  
  
string foo( );  
  
  
void bar(string & s);   
 
 

The following expression is invalid:

 
 

  
  
bar(foo( ));  
  
  
bar("hello world"); 
 
 

The reason is that both the foo () and "hello world" strings generate a temporary object. In C ++, these temporary objects are of the const type. Therefore, the above expression tries to convert a const type object to a non-const type, which is invalid.

The referenced parameter should be defined as const if it can be defined as const.