Summary of C/C ++ written examination and interview questions

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

Int func (X)
{
Int countx = 0;
While (X)
{
Countx ++;
X = x & (x-1 );
}
Return countx;
}

Assume 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

Int;
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.

5. Use "Reference" as the format, benefits, and rules of the function return value type?

Format: type identifier & function name (parameter list and type description) {// function body}

Benefit: do not generate a copy of the returned value in the memory. (Note: For this reason, it is not advisable to return a reference to a local variable. As the survival of the local variable ends, the corresponding reference will also become invalid, resulting in a runtime error!

Note:

(1) References to local variables cannot be returned. For details, refer to item 31 of Objective C ++ [1. The main reason is that local variables will be destroyed after the function returns, so the returned reference becomes a reference of "no finger", and the program enters the unknown state.

(2) You cannot return a reference to the memory allocated by the new function. For details, refer to item 31 of Objective C ++ [1. Although there is no passive destruction of local variables, this situation (returning a reference to the memory allocated by the new function) faces other embarrassing situations. For example, if a reference returned by a function only appears as a temporary variable and is not assigned to an actual variable, the space pointed to by the reference (allocated by new) cannot be released, cause memory leak.

(3) You can return a reference to a class member, but it is best to use Const. This principle can be referred to item 30 of Objective C ++ [1. The main reason is that when an object attribute is associated with a business rule, its value assignment is often related to some other attributes or the state of the object, therefore, it is necessary to encapsulate the value assignment operation in a business rule. If other objects can obtain the non-constant reference (or pointer) of this attribute, a simple value assignment to this attribute will damage the integrity of business rules.

(4) The function of declaring the returned value of stream operator overload as "Reference:

Stream operators <and>, which are often used consecutively, for example, cout <"hello" <Endl; therefore, the return value of these two operators should be a stream reference that still supports these two operators. Other optional solutions include returning a stream object and returning a stream object pointer. However, for a returned Stream object, the program must re-(copy) to construct a new stream object. That is to say, two consecutive <operators are actually for different objects! This is unacceptable. If a stream pointer is returned, the <operator cannot be used consecutively. Therefore, returning a stream object reference is the only choice. This unique choice is critical. It illustrates the importance of reference and is irreplaceable. Maybe this is why the concept is introduced in C ++. Value assignment operator =. This operator can be used continuously like a stream operator, for example, x = J = 10; or (x = 10) = 100; the return value of the value assignment operator must be a left value, so that the value can be assigned. Therefore, it is referenced as the only return value choice of this operator.

Example 3

# I nclude <iostream. h>
Int & put (int n );
Int Vals [10];
Int error =-1;
Void main ()
{
Put (0) = 10; // use the put (0) function value as the left value, equivalent to Vals [0] = 10;
Put (9) = 20; // use the put (9) function value as the left value, which is equivalent to Vals [9] = 20;
Cout <Vals [0];
Cout <Vals [9];
}
Int & put (int n)
{
If (n> = 0 & n <= 9) return Vals [N];
Else {cout <"subscript error"; Return Error ;}

Bool check (const node * head)
{
If (Head = NULL) return false;
Node * low = head, * fast = head-> next;
While (fast! = NULL & fast-> next! = NULL)
{
Low = low-> next;
Fast = fast-> next;
If (Low = fast) return true;
}
Return false