C++ 是否允许将任何整数文字隐式转换为 short int？
Does C++ allow any integer literal to be implicitly converted to a short int?
int main()
{
short n1 = 8ll; // no warning
// warning C4305: 'initializing': truncation from '__int64' to 'short'
// warning C4309: 'initializing': truncation of constant value
short n2 = 88888ll;
}
我的编译器是 Visual Studio 2017.
根据cppref：
The type of the integer literal is the first type in which the value
can fit, from the list of types which depends on which numeric base
and which integer-suffix was used.
后缀为ll的整数字面量应为long long int；所以 short n1 = 8ll 应该像 short n2 = 88888ll 一样触发警告。
C++ 是否允许将任何整数文字隐式转换为 short int（如果它足够小）？
标准允许任意两个整数类型之间的隐式转换，无论它们的值如何。
编译器警告与代码是否合法无关；编译器只是在您的代码可能未按照您的要求执行时发出警告。
在您的特定情况下，n1 将是 8，而 n2 将具有实现定义的值。这两个赋值都是合法的 C++，但后者可能不是您想要的。
相关标准语：
A prvalue of an integer type can be converted to a prvalue of another integer type. A prvalue of an unscoped
enumeration type can be converted to a prvalue of an integer type.
If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source
integer (modulo 2n where n is the number of bits used to represent the unsigned type). [ Note: In a two’s
complement representation, this conversion is conceptual and there is no change in the bit pattern (if there
is no truncation). — end note ]
If the destination type is signed, the value is unchanged if it can be represented in the destination type (and
bit-field width); otherwise, the value is implementation-defined.
N4141 中的 4.7/1-3
是的，整数可以隐式转换。这些是 C++ 标准草案 N4296 中的规则：
4.7 Integral conversions
1 A prvalue of an integer type can be converted to a prvalue of
another integer type. A prvalue of an unscoped enumeration type can be
converted to a prvalue of an integer type.
2 If the destination type
is unsigned, the resulting value is the least unsigned integer
congruent to the source integer (modulo 2n where n is the number of
bits used to represent the unsigned type).
[ Note: In a two’s
complement representation, this conversion is conceptual and there is
no change in the bit pattern (if there is no truncation). —end note ]
3 If the destination type is signed, the value is unchanged if it can
be represented in the destination type; otherwise, the value is
implementation-defined.
4 If the destination type is bool, see 4.12.
If the source type is bool, the value false is converted to zero and
the value true is converted to one.
5 The conversions allowed as
integral promotions are excluded from the set of integral conversions.
int main()
{
short n1 = 8ll; // no warning
// warning C4305: 'initializing': truncation from '__int64' to 'short'
// warning C4309: 'initializing': truncation of constant value
short n2 = 88888ll;
}
我的编译器是 Visual Studio 2017.
根据cppref：
The type of the integer literal is the first type in which the value can fit, from the list of types which depends on which numeric base and which integer-suffix was used.
后缀为ll的整数字面量应为long long int；所以 short n1 = 8ll 应该像 short n2 = 88888ll 一样触发警告。
C++ 是否允许将任何整数文字隐式转换为 short int（如果它足够小）？
标准允许任意两个整数类型之间的隐式转换，无论它们的值如何。
编译器警告与代码是否合法无关；编译器只是在您的代码可能未按照您的要求执行时发出警告。
在您的特定情况下，n1 将是 8，而 n2 将具有实现定义的值。这两个赋值都是合法的 C++，但后者可能不是您想要的。
相关标准语：
A prvalue of an integer type can be converted to a prvalue of another integer type. A prvalue of an unscoped enumeration type can be converted to a prvalue of an integer type.
If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2n where n is the number of bits used to represent the unsigned type). [ Note: In a two’s complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation). — end note ]
If the destination type is signed, the value is unchanged if it can be represented in the destination type (and bit-field width); otherwise, the value is implementation-defined.
N4141 中的 4.7/1-3
是的，整数可以隐式转换。这些是 C++ 标准草案 N4296 中的规则：
4.7 Integral conversions
1 A prvalue of an integer type can be converted to a prvalue of another integer type. A prvalue of an unscoped enumeration type can be converted to a prvalue of an integer type.
2 If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2n where n is the number of bits used to represent the unsigned type).
[ Note: In a two’s complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation). —end note ]
3 If the destination type is signed, the value is unchanged if it can be represented in the destination type; otherwise, the value is implementation-defined.
4 If the destination type is bool, see 4.12. If the source type is bool, the value false is converted to zero and the value true is converted to one.
5 The conversions allowed as integral promotions are excluded from the set of integral conversions.