第8课 列表初始化（3）_防止类型收窄、explicit关键字

#include <iostream>
#include <vector>
using namespace  std;

int main()
{
    int x1(5.3);   //ok，x1=5
    int x2 = 5.3;  //ok，x2=5
    int x3 {5.3};  //error：narrowing，gcc给的是warning
    int x4 = {5.3};//同上
    char c1{7};    //ok
    char c2{99999};//error：narrowing，gcc给的是warning
    std::vector<int> v1{1,2,3,4,5}; //ok
    std::vector<int> v2{1,2,3,4,5.6}; //error：narrowing，gcc给的是warning
    
    float fa = 1e40;  //ok，double->float
    float fb = {1e40};//error，double->float，超过float能表示的范围
    float fc = (unsigned long long)-1; //将-1（0xFFFFFFFF）强转，再隐式转成float
    float fd = {(unsigned long long)-1}; //error
    float fe = (unsigned long long)1; //将1强转，再隐式转成float
    float ff = {(unsigned long long)1}; //ok
    
    const int x = 1024, y = 1; //注意x，y被const修饰
    char c = x;  //ok
    char d = {x};//error
    char e = y;  //ok
    char f = {y};//ok。如果y为int型，则该行会出错。但由于加了const并且值为1。编译器
                 //认为这样的转换是安全的。
    
    
    return 0;
}
/*
*******************gcc上的测试结果*********************
e:\Study\C++11\8>g++ -std=c++11 test1.cpp
test1.cpp: In function ‘int main()‘:
test1.cpp:9:13: error: narrowing conversion of ‘5.2999999999999998e+0‘ from ‘double‘ to ‘int‘ inside { } [-Wnarrowing]
test1.cpp:10:15: error: narrowing conversion of ‘5.2999999999999998e+0‘ from ‘double‘ to ‘int‘ inside { } [-Wnarrowing]
test1.cpp:12:15: error: narrowing conversion of ‘99999‘ from ‘int‘ to ‘char‘ inside { } [-Wnarrowing]
test1.cpp:14:33: error: narrowing conversion of ‘5.5999999999999996e+0‘ from ‘double‘ to ‘int‘ inside { } [-Wnarrowing]
test1.cpp:17:18: error: narrowing conversion of ‘1.0e+40‘ from ‘double‘ to ‘float‘ inside { } [-Wnarrowing]
test1.cpp:19:36: error: narrowing conversion of ‘18446744073709551615ull‘ from ‘long long unsigned int‘ to ‘float‘ inside { } [-Wnarrowing]
test1.cpp:24:11: warning: overflow in implicit constant conversion [-Woverflow]
test1.cpp:25:13: error: narrowing conversion of ‘1024‘ from ‘int‘ to ‘char‘ inside { } [-Wnarrowing]
  
*******************vc2015上的测试结果*********************
test1.cpp(9): error C2397: 从“double”转换到“int”需要收缩转换
test1.cpp(10): error C2397: 从“double”转换到“int”需要收缩转换
test1.cpp(12): error C2397: 从“int”转换到“char”需要收缩转换
test1.cpp(14): error C2398: 元素“5”: 从“double”转换到“int”需要收缩转换
test1.cpp(17): error C2397: 从“double”转换到“float”需要收缩转换
test1.cpp(19): error C2397: 从“unsigned __int64”转换到“float”需要收缩转换
test1.cpp(25): error C2397: 从“const int”转换到“char”需要收缩转换
*/

  1 #include <iostream>
  2 using namespace std;
  3 
  4 class Test
  5 {
  6 public:
  7     Test(int a, int b)
  8     {
  9         cout <<"Test(int a, int b)" << endl;
 10     }
 11     
 12     Test(initializer_list<int>)
 13     {
 14         cout <<"Test(initializer_list<int>)" << endl;
 15     }
 16     
 17     explicit Test(int a, int b, int c)
 18     {
 19         cout <<"explicit Test(int a, int b, int c)" << endl;
 20     }
 21 };
 22 
 23 void func(const Test& t)
 24 {
 25     
 26 }
 27 
 28 struct Complex
 29 {
 30     int real, imag;
 31     
 32     //explicit 
 33     Complex(int re, int im = 0):real(re), imag(im){}
 34     
 35     Complex operator+(const Complex& x)
 36     {
 37         return Complex((real+x.real),(imag+x.imag));
 38     }
 39 };
 40 
 41 int main()
 42 {
 43     Complex c1(12, 5);
 44     Complex c2 = c1 + 5;//会试图将5转换为Complex类型，所以会隐式调用构造
 45                         //函数。为了阻止这种行为，可以在构造函数前加explicit
 46     
 47     //1.由于Test带有一个initializer_list参数的构造函数，因此，下列
 48     //用{}初始化的对象都会直接调用该构造函数。
 49     //2.如果注释掉上述构造函数，则编译器会将{...}分解并传给相应的
 50     //构造函数，如果找不到相应的带多参的构造函数，则直接报错。
 51     Test t1(77, 5);       //Test(int a, int b)
 52     Test t2{77, 5};       //Test(initializer_list<int>)
 53     Test t3{77, 5, 42};   //Test(initializer_list<int>)
 54     Test t4 = {77, 5};    //Test(initializer_list<int>)
 55     Test t5 = {77, 5, 42};//Test(initializer_list<int>)
 56     Test t6(77, 5, 42);   //ok，显式调用explicit Test(int a, int b, int c)
 57     
 58     func({47, 11});           //Test(initializer_list<int>)
 59     func({47, 11, 3});        //Test(initializer_list<int>)
 60     func(Test{47, 11});       //Test(initializer_list<int>)
 61     func(Test{47, 11, 3});    //Test(initializer_list<int>)
 62     
 63     Test t11{77, 5, 42, 500};   //Test(initializer_list<int>)
 64     Test t12 = {77, 5, 42, 500};//Test(initializer_list<int>)
 65     Test t13 {10};  //Test(initializer_list<int>)
 66     
 67     return 0;
 68 }
 69 /*测试结果：
 70 **************************不注释Test中带有一个initializer_list参数的构造函数***************************
 71 e:\Study\C++11\8>g++ -std=c++11 test2.cpp
 72 e:\Study\C++11\8>a.exe
 73 Test(int a, int b)
 74 Test(initializer_list<int>)
 75 Test(initializer_list<int>)
 76 Test(initializer_list<int>)
 77 Test(initializer_list<int>)
 78 explicit Test(int a, int b, int c)
 79 Test(initializer_list<int>)
 80 Test(initializer_list<int>)
 81 Test(initializer_list<int>)
 82 Test(initializer_list<int>)
 83 Test(initializer_list<int>)
 84 Test(initializer_list<int>)
 85 Test(initializer_list<int>)
 86 
 87 **************************不注释Test中带有一个initializer_list参数的构造函数***************************
 88 e:\Study\C++11\8>g++ -std=c++11 test2.cpp -fno-elide-constructors
 89 test2.cpp: In function ‘int main()‘:
 90 test2.cpp:55:22: error: converting to ‘Test‘ from initializer list would use explicit constructor ‘Test::Test(int, int, int)‘
 91   Test t5 = {77, 5, 42};
 92 
 93 test2.cpp:59:18: error: converting to ‘const Test‘ from initializer list would use explicit constructor ‘Test::Test(int, int, int)‘
 94   func({47, 11, 3});  
 95 
 96 test2.cpp:63:25: error: no matching function for call to ‘Test::Test(<brace-enclosed initializer list>)‘
 97   Test t11{77, 5, 42, 500}; 
 98 
 99 test2.cpp:64:28: error: could not convert ‘{77, 5, 42, 500}‘ from ‘<brace-enclosed initializer list>‘ to ‘Test‘
100   Test t12 = {77, 5, 42, 500};
101 
102 test2.cpp:65:14: error: no matching function for call to ‘Test::Test(<brace-enclosed initializer list>)‘
103   Test t13 {10};
104 */