Upgrade from C to C ++ to c ++

Upgrade from C to C ++ to c ++

I. Relationship between C and C ++

1. C ++ inherits all the features of C Language

2. C ++ provides more syntaxes and features based on C.

3. The objective of C ++ is to unify the running efficiency and development efficiency.

　　

C-> C Language

+-> Object-oriented support

+-> Type enhancement and function Enhancement

II. C ++ emphasizes the "practicability" of the language. All variables can be defined as needed.

In C, it is legal to repeatedly define multiple global variables with the same name.

In C ++, multiple global variables with the same name cannot be defined.

Multiple global variables with the same name in C will be connected to the same address space in the global data zone.

C ++ directly rejects this ambiguity.

3. C ++ enhancements to C

1. Processing of const Constants by C ++ Compiler

A. When a constant declaration is encountered, a constant is placed in the symbol table.

B. If constants are found during compilation, they are directly replaced with values in the symbol table.

C. If the extern or & operator is used for const during compilation, the storage space is allocated to the corresponding constant.

Const constants in C language are read-only constants and have their own storage space.

Const constant in C ++

Storage space may be allocated.

# When the const constant is global and needs to be used in other files

# When the & operator is used to obtain the address of the const constant

#include <stdio.h>int main(int argc, char *argv[]){    const int c = 0;    int* p = (int*)&c;        printf("Begin...\n");        *p = 5;        printf("c = %d\n", c);        printf("End...\n");        printf("Press enter to continue ...");    getchar();        return 0;}

The output is C = 0;

2. const summary in C ++

Const constants in a and C ++ are similar to macro definitions.

Const int c = 5; approximately equal to # define c 5

Const constants in B. C ++ are different from macro definitions.

Const constants are processed by the compiler and provide type checks and scope checks.

Macro definition is processed by a pre-processor and replaced simply by text.

#include <stdio.h>void f(){    #define a 3    const int b = 4;}void g(){    printf("a = %d\n", a);    //printf("b = %d\n", b);}int main(int argc, char *argv[]){    f();    g();        printf("Press enter to continue ...");    getchar();        return 0;}

Output: a = 3; B returns an error.

3. Enhanced struct type

A. the struct in C Language defines a set of variables. The C compiler does not regard this as a new type.

Struct in B and C ++ is a new type definition declaration.

Struct Student

(

Const char * name;

Int age;

)

Student s1;

Student s2;

4. Changes in the register keyword

A. The register keyword requests the "compiler" to store local variables in registers.

Register variable address cannot be obtained in C Language

B. The register keyword is still supported in C ++.

# The C ++ compiler has its own optimization method. It may also be optimized without using register.

# The address of the register variable can be obtained in C ++.

When the C ++ compiler finds that the address of the register variable is required in the program, the register Declaration of the variable becomes invalid (because the C ++ compiler automatically optimizes the variable ).

Early C language compilers do not optimize the code, so the register variable is a good supplement.

　　　　

5. All variables and functions in C ++ must be of the C type.

A. The default type in C language is invalid in C ++.

F (I)

{

Printf ("I = % d \ n", I );

}

G ()

{

Return 5;

}

B. The function in C language contains a non-argument function.

Void f (void)

The function in C ++ is a nonargument function.

Both void f () and void f (void) are supported.

4. Summary

1. C ++ emphasizes practicality and can declare variables anywhere

2. register in C ++ is only a backward compatible function, and the C ++ compiler can perform better variable optimization.

3. const in C ++ is a real constant instead of a read-only constant.

4. C ++ emphasizes the type, and any program element must display the specified type.