C++ function inlining

1. Replace macro code with inlining

The C++ language supports function inlining, whose purpose is to improve the execution efficiency (speed) of the function.

In C programs, macro codes can be used to improve execution efficiency. The macro code itself is not a function, but it behaves like a function. The preprocessor replaces function calls by copying macro codes, eliminating the need for pushing parameters onto the stack, generating CALL calls in assembly language, returning parameters, and executing return, thus improving speed. The biggest disadvantage of using macro code is that it is error-prone. The preprocessor often produces unexpected side effects when copying macro code.

For example the

#define MAX(a, b)         (a) > (b) ? (a) : (b)

statement

result = MAX(i, j) + 2 ;

will be interpreted by the preprocessor as

result = (i) > (j) ? (i) : (j) + 2 ;

due to the operator ' + 'Than operator' :' has a higher priority, so the above statement is not equivalent to the expected

result = ( (i) > (j) ? (i) : (j) ) + 2 ;

If you rewrite the macro code as

#define MAX(a, b) ( (a) > (b) ? (a) : (b) )

, you can solve the problem caused by priority mistake. But even using modified macro code is not foolproof, for example the statement

result = MAX(i++, j);

will be interpreted by the preprocessor as

result = (i++) > (j) ? (i++) : (j);

For C++, there is another disadvantage of using macro code : Private data members of the class cannot be operated, which means that the macro code is basically for public or global operations.

#Let’s see how C++’s “function inlining” works. For any inline function, the compiler places the function's declaration (including name, parameter types, and return value type) in the symbol table. If the compiler finds no error in the inline function, the code for the function is also placed in the symbol table. When calling an inline function, the compiler first checks whether the call is correct (performing type safety checks, or performing automatic type conversion, of course the same for all functions). If correct, the code for the inline function will directly replace the function call, thus eliminating the overhead of the function call. This process is significantly different from preprocessing, because the preprocessor cannot perform type safety checks or perform automatic type conversions. If the inline function is a member function, the address of the object (this) will be placed in the appropriate place, which is something that the preprocessor cannot do.

#The function inlining mechanism of C++ language not only has the efficiency of macro code, but also increases security, and can freely operate the data members of the class. Therefore, in C++ programs, all macro codes should be replaced by inline functions. "assert" is probably the only exception. assert is a macro that only works in the Debug version. It is used to check for situations that "shouldn't" happen. In order not to cause differences between the Debug and Release versions of the program, assert should not have any side effects. If assert is a function, since the function call will cause changes in memory and code, there will be a difference between the Debug version and the Release version. So assert is not a function, but a macro.

2. Programming style of inline function

The keyword inline must be placed together with the function definition body. To make a function inline, simply putting inline in front of the function declaration has no effect. The function Foo in the following style cannot become an inline function:

inline void Foo(int x, int y);   // inline 仅与函数声明放在一起,不起任何作用  
void Foo(int x, int y)  
{  
…  
}

And the function Foo in the following style becomes an inline function:

void Foo(int x, int y);    
inline void Foo(int x, int y) // inline 与函数定义体放在一起  
{  
…  
}

So, inline It is a "keyword for implementation" rather than a "keyword for declaration". Generally, users can read the declaration of a function, but cannot see the definition of the function. Although the inline keyword is added before the declaration and definition body of inline functions in most textbooks, I think inline should not appear in the declaration of a function. Although this detail will not affect the functionality of the function, it reflects a basic principle of high-quality C++/C programming style: declaration and definition cannot be confused, and users do not need to, and should not, know whether a function needs to be inlined.

Member functions defined in the class declaration will automatically become inline functions, for example

class A  
{  
public:  
    void Foo(int x, int y) { … }   // 自动地成为内联函数  
}

will Although placing the definition body of the member function in the class declaration brings convenience in writing, it is not a good programming style. The above example should be changed to:

// 头文件  
class A  
{  
public:  
    void Foo(int x, int y)；  
}  
// 定义文件  
inline void A::Foo(int x, int y)  
{  
…  
}

3. Use inlining with caution

Inlining can improve the execution efficiency of functions. Why not define all functions as inline functions?

#If all functions are inline functions, is the keyword "inline" still needed?

Inline is at the expense of code expansion (duplication), and only saves the overhead of function calls, thus improving the execution efficiency of the function. If the time to execute the code in the function body is larger than the overhead of function calls, then the efficiency gains will be very small. On the other hand, each inline function call requires copying the code, which will increase the total code size of the program and

consume more memory space. It is not appropriate to use inlining in the following situations:

(1) If the code in the function body is relatively long, using inlining will result in higher memory consumption costs.

# (2) If a loop occurs in the function body, the time to execute the code in the function body is greater than the cost of the function call.

The constructors and destructors of classes are easily misunderstood as using inline is more efficient. Beware of constructors and destructors. Functions may hide some behavior, such as "secretly" executing the constructors and destructors of base classes or member objects. So don't just put the definitions of constructors and destructors in the class declaration.

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