为什么初始化列表构造函数参数时，异常发生？

Question

template <typename T> 
Blob<T>::Blob(std::initializer_list<T> il) try : 
data(std::make_shared<std::vector<T>>(il)) { 
/* empty body */ 
} catch(const std::bad_alloc &e) { handle_out_of_memory(e); } 

C++入门第五版779页说

Notice that the keyword try appears before the colon that begins the constructor initializer list and before the curly brace that forms the (in this case empty) constructor function body. The catch associated with this try can be used to handle exceptions thrown either from within the member initialization list or from within the constructor body. It is worth noting that an exception can happen while initializing the constructor’s parameters. Such exceptions are not part of the function try block. The function try block handles only exceptions that occur once the constructor begins executing. As with any other function call, if an exception occurs during parameter initialization, that exception is part of the calling expression and is handled in the caller’s context.

我很困惑，不能想到的，当它发生在那里/的情况下，任何人都可以给我 一个例子吗？

Answer 1

下面是一个例子：

struct S 
{ 
    S(char *); 
}; 

int main() 
{ 
    S s(new char[0x7FFFFFFF]); 
} 

的new char[0x7FFFFFFF]可能会引发内存外的一个例外。

Answer 2

如果初始化参数时内存不足。

Answer 3

例如：抛出异常：读取访问冲突。

#include <iostream> 

using namespace std; 

class X 
{ 
    int a; 
    int b; 
    int* c; 

    public: 
     X(int a, int b, int* c = nullptr) try : a(a), b(b=c[0]) 
     { 
     } 
     catch (...) 
     { 
      cout << "Exception occured"; 
     } 
}; 

int main() 
{ 
    X v(1,0); 

    return 0; 
} 

Message: c was nullptr.

Answer 4

当一个例外是投一个构造函数调用可能会发生这样的事;隐式或显式。

例如：

class spam { 
public: 
    spam(int eggs) { 
     throw std::exception(); 
    } 
}; 

class foo { 
public: 
    spam unused; 
    const int q; 
    foo() try : unused(1), q(123) { 

    } catch(std::exception& e) { std::cout << "spamspamspam\n";} 
}; 

int main() { 
    try { 
     foo f; // exception thrown 
    } catch (std::exception& e) { 
     std::cout << "It didn't stop there!\n"; 
    } 
    return 0; 
} 

打印在终端上 “！spamspamspam \ n它并没有就此停止\ n”，然后退出与中止。请注意，该异常可以被捕获，但之后会继续传播。这是因为在投掷者之后初始化的任何成员实际上都是而不是已初始化，并且const成员或引用（如const int q在我的示例中）在语法上不可能在稍后的时间点进行初始化。所以答案是完全中止构造函数调用。