比如下面這個問題

long double ld=3.1415926536;
int a{ld},b={ld}; //錯誤，轉換未執行，因為存在丟失資訊的危險
int c(ld),d=(ld); //正確，轉化執行，且確實丟失了部分值

為什麼會提示 “a”本地函式定義是非法的，而c，d卻又沒問題呢？這跟a用花括號定義有什麼聯絡？

()是呼叫了型別的建構函式初始化，對於內建型別來說，編譯器有預設的建構函式，類似這樣：
struct int {
int (const int&);
int (const double&);
...
four bytes data;
};
變數c的初始化，就是呼叫了其中的一個

建構函式（double），所以不會出現警告。

而 {}初始化的方法，僅被最新的C++11標準支援，有個專門的術語：initializer-list。
這種方法沒有使用建構函式，所以凡是能導致精度降低、範圍變窄等等的初始化情況，統稱為 narrowing conversion，編譯器都會警告，narrowing conversion 具體的情況有：
A narrowing conversion is an implicit conversion
— from a floating-point type to an integer type, or
— from long double to double or float, or from double to float, except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented (even if it cannot be represented exactly), or
— from an integer type or unscoped enumeration type to a floating-point type, except where the source is a constant expression and the actual value after conversion will

fit into the target type and will produce the original value when converted back to the original type, or
— from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression and the actual value after conversion will

fit into the target type and will produce the original value when converted back to the original type.

轉自：https://zhidao.baidu.com/question/1668775549685301747.html