C++ Primer 课后习题解析

第2章 变量和基本类型

Exercise 2.9

Explain the following definitions. For those that are illegal, explain what’s wrong and how to correct it.

• (a) std::cin >> int input_value;

• (b) int i = { 3.14 };

• (c) double salary = wage = 9999.99;

• (d) int i = 3.14;

(a): error: expected '(' for function-style cast or type construction.（？）

int input_value = 0;
std::cin >> input_value;

(b):when you compile the code without the argument "-std=c++11", you will get the warning below: warning: implicit conversion from 'double' to 'int' changes value from 3.14 to 3.

when you compile the code using "-std=c+11", you will get a error below: error: type 'double' cannot be narrowed to 'int' in initializer list

conclusion: Obviously, list initialization becomes strict in c++11.

double i = { 3.14 };

(c): --if you declared 'wage' before, it's right. Otherwise, you'll get a error: error: use of undeclared identifier 'wage'

double wage;
double salary = wage = 9999.99;

(d): ok: but value will be truncated(截断).

double i = 3.14;

Exercise 2.10

What are the initial values, if any, of each of the following variables?

std::string global_str;
int global_int;
int main()
{
  int local_int;
  std::string local_str;
}

global_str is global variable, so the value is empty string. 

global_int is global variable, so the value is zero.

local_int is a local variable which is not uninitialized, so it has a undefined value. 

local_str is also a local variable which is not uninitialized, but it has a value that is defined by the class. So it is empty string.

PS: please read P44 in the English version, P40 in Chinese version to get more.

The note: Uninitialized objects of built-in type defined inside a function body have a undefined value. Objects of class type that we do not explicitly明确地） inititalize have a value that is defined by class.

定义于函数内部体内的内置类型的对象如果没有初始化，则其值 未定义。

类的对象如果没有显式地初始化，则其值 由类决定。