C++ Primer 读书笔记2(二)

Unlike subscripts for vector and string,the index of the built-in subscript operator is not an unsigned type.

string s1 = "A string example"; 
string s2 = "A different string"; 
if (s1 < s2)  // false: s2 is less than s1

const char ca1[] = "A string example"; 
const char ca2[] = "A different string"; 
if (ca1 < ca2)  // undefined: compares two unrelated addresses

For most applications, in addition to beingsafer, it is also more efficient to use library strings rather than C-stylestrings.

string s("Hello World");  // s holds Hello World
char *str = s; // error: can't initialize a char* from a string 
const char *str = s.c_str(); // ok

The array returned by c_str is notguaranteed to be valid indefinitely. Any subsequent use of s that might changethe value of s can invalidate this array.

If a program needs continuing access to thecontents of the array returned by str(), the program must copy the arrayreturned by c_str.

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;    
} 
}
《==》
size_t cnt = 0; 
for (auto &row : ia)        // for every element in the outer array
for (auto &col : row) { // for every element in the inner array
        col = cnt;          // give this element the next value
        ++cnt;              // increment cnt
}

In the previous example, we used referencesas our loop control variables because we wanted to change the elements in thearray. However, there is a deeper reason for using references. As an example,consider the following loop:

for (const auto &row : ia)  // for every element in the outer array
for (auto col : row)    // for every element in the inner array
        cout << col << endl;

This loop does not write to the elements,yet we still define the control variable of the outer loop as a reference. Wedo so in order to avoid the normal array to pointer conversion.

Note: To use a multidimensional array in a range for, the loop controlvariable for all but the innermost array must be references.

Exercise 3.44: Rewrite the programs fromthe previous exercises using a type alias for the type of the loop controlvariables.

Exercise 3.45: Rewrite the programsagain, this time using auto.

	const size_t row = 3;
	const size_t col = 4;
	int arr[row][col];
	int cnt = 0;

	//版本1
	for (int(&p)[4] : arr)
	{
		for (int &q : p)
		{
			q = cnt++;
		}
	}
	for (int(&p)[4] : arr)
	{
		for (int q : p)
		{
			cout << q << " ";
		}
		cout << endl;
	}
	cout << endl;

	//版本2
	for (size_t i = 0; i != row; ++i)
		for (size_t j = 0; j != col; ++j)
			arr[i][j] = i * 4 + j;

	for (size_t i = 0; i != row; ++i)
	{
		for (size_t j = 0; j != col; ++j)
		{
			cout << arr[i][j] << " ";
		}
		cout << endl;
	}
	
	//版本3
	for (int(*p)[4] = arr; p != arr + 3; ++p)
	{
		for (int *q = *p; q != *p + 4; ++q)
		{
			*q = cnt++;
		}
	}

	for (int(*p)[4] = arr; p != arr + 3; ++p)
	{
		for (int *q = *p; q != *p + 4; ++q)
		{
			cout << *q << " ";
		}
		cout << endl;
	}

	//练习3.44
	using int_arr = int[4];
	for (int_arr *p = arr; p != arr + 3; ++p)
	{
		for (int *q = *p; q != *p + 4; ++q)
		{
			*q = cnt++;
		}
	}

	for (int_arr *p = arr; p != arr + 3; ++p)
	{
		for (int *q = *p; q != *p + 4; ++q)
		{
			cout << *q << " ";
		}
		cout << endl;
	}

	//练习3.45
	for (auto &p : arr)
		for (auto &q : p)
			q = cnt++;

	for (auto &p: arr)
	{
		for (auto& q : p)
		{
			cout << q << " ";
		}
		cout << endl;
	}