c - 字符表达式的大小

Question

有人能解释一下 sizeof(char expression) 是如何变成 sizeof(int) 的吗？

int main()
{
char a, b;
printf("%d\n", sizeof(a+b));
return 0;
}

程序打印 4 作为输出。我期望它是 sizeof(char) 即 1

Answer 1

整数提升。

当小整数类型(小于int，如char、short等)在算术表达式中时，自动提升到 int。

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C11 §6.3.1.1 Boolean, characters, and integers

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned
int. These are called the integer promotions. ⁵⁸⁾

在脚注中:

58) The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the shift operators, as specified by their respective subclauses.