《C++ Primer（第五版）》知识巩固

运行平台：ubuntu 12.04/GCC 4.8.0

 

第二章：基本内置类型

 1.decltype类型指示符

当我们从表达式的类型来推断要定义的类型时，可以使用decltype()来解析；decltype与auto不同，decltype应用于变量，返回该变量的类型。

	string s("Hello World!!!");
	// punct_cnt has the same type that s.size returns
	decltype(s.size()) punct_cnt = 0; 

2.转义序列

\xxx 八进制

\x(xxx) 十六进制

unsigned char c = -1;   // assuming 8-bit chars, c has value 255
	i = c;  // the character with value 255 is an unprintable character
	        // assigns value of c (i.e., 255) to an int
	std::cout << i << std::endl; // prints 255

	unsigned u = 10;
	int i = -42;
	std::cout << i + i << std::endl;  // prints -84
	std::cout << u + i << std::endl;  // if 32-bit ints, prints 4294967264

3.getline()的使用

string line;

	// read input a line at a time until end-of-file
	while (getline(cin, line))
		cout << line << endl;

 4.str.size()返回为string::size_type类型，这是标准库与机器无关的特性之一。

如下面这个例子，输入大小在0-15之间的数字，求其对应的十六进制符号：

const string hexdigits = "0123456789ABCDEF";  // possible hex digits

	cout << "Enter a series of numbers between 0 and 15"
	     << " separated by spaces.  Hit ENTER when finished: " 
	     << endl;
	string result;        // will hold the resulting hexify'd string

	string::size_type n;  // hold numbers from the input
	while (cin >> n)
		if (n < hexdigits.size())    // ignore invalid input
			result += hexdigits[n];  // fetch the indicated hex digit

	cout << "Your hex number is: " << result << endl;

 而与之相关的则是size_t类型，size_t是标准C库中定义的，应为unsigned int，在64位系统中为 long unsigned int。

#include <cstddef>
using std::size_t;

constexpr size_t rowCnt = 3, colCnt = 4;
	int ia[rowCnt][colCnt];   // 12 uninitialized elements 
	
    // for each row
    for (size_t i = 0; i != rowCnt; ++i) {
        // for each column within the row
        for (size_t j = 0; j != colCnt; ++j) {
            // assign the element's positional index as its value
            ia[i][j] = i * colCnt + j;   
		}
	}

 第四章：表达式

1.值的溢出

思考下面运算的结果：

short short_value = 32767; // max value if shorts are 16 bits

	short_value += 1; // this calculation overflows
	cout << "short_value: " << short_value << endl;

 第六章：函数

1.返回数组指针的函数，这三种表达方式，留意：

int *p = elemPtr(6);         // p points to an int
	int (*arrP)[5] = arrPtr(5);  // arrP points to an array of five ints
	int (&arrR)[5] = arrRef(4);  // arrR refers to an array of five ints

/ two arrays
int odd[] = {1,3,5,7,9};
int even[] = {0,2,4,6,8};

// function that returns a pointer to an int in one of these arrays
int *elemPtr(int i)
{
	// returns a pointer to the first element in one of these arrays
	return (i % 2) ? odd : even;  
}
	
// returns a pointer to an array of five int elements
decltype(odd) *arrPtr(int i)
{
	return (i % 2) ? &odd : &even; // returns a pointer to the array 
}

// returns a reference to an array of five int elements
int (&arrRef(int i))[5]
{
	return (i % 2) ? odd : even;
}

 2.含有可变参数的函数

在C11中提供了两种方法：如果参数类型相同，可以传递一个名为initializer_list的标准库类型；如果类型不同，可以使用可变参数模版，见16章。使用方法如下：

// overloaded version takes only a list of strings
void error_msg(initializer_list<string> il)
{
	for (auto beg = il.begin(); beg != il.end(); ++beg)
		cout << *beg << " " ;
	cout << endl;
}

string expected = "description", actual = "some other case";
	initializer_list<int> li = {0,1,2,3};

	// expected, actual are strings 
	if (expected != actual)
		error_msg({"functionX", expected, actual});
	else
		error_msg({"functionX", "okay"});

 3.函数指针

C11中尾置返回类型，如声明如下：

// use trailing return type
auto getFcn(const string&) -> string::size_type(*)(const string&, const string&);

 　　而函数指针的另外两种声明定义，一种是（）括号解析，另一种则是借用decltype确定返回类型为函数指针类型，使用如下：

decltype(sumLength) *getFcn(const string &);

// direct definition
string::size_type (*getFcn(const string&))(const string&, const string&);

// define getFcn
decltype(sumLength)* 
getFcn(const string &fetch)
{
	if (fetch == "sum")
		return sumLength;
	return largerLength;
}