如何printf和联合区分beetween float和double [英] How does printf and co differentiate beetween float and double
问题描述
由于它不是强类型的我还以为它只是选择了正确的内存大小，跨preTED它根据参数的类型。但是，float和double都使用％f和他们是不同的大小。
Since it isn't strongly typed I thought it just picked the right memory size and interpreted it based on the type of argument. But float and double both use %f and they are different sizes.
P.S。
我可以看到通过价值复制到一个临时铸造推广如何（这是正确的？）可能工作
但它是如何进行工作scanfs / sscanf的？
P.S. I can see how promotion via copying the value to a temp and casting(is this right?) might work but how does it work for scanfs/sscanf?
推荐答案
它不区分。这是不可能接收浮动作为可变参数：任何浮动参数您提供的第一个晋升为双击。
It doesn't differentiate. It's not possible to receive a float as a vararg: any float argument that you provide is first promoted to double.
6.5.2.2/6定义默认参数提升，和/ 7指出默认参数提升应用到尾随参数，即通过可变参数 ... 。
6.5.2.2/6 defines "default argument promotions", and /7 states that default argument promotions are applied to "trailing arguments", that is varargs denoted by ....
它是如何为scanfs / sscanf的工作？
how does it work for scanfs/sscanf?
的％F 的格式为： scanf函数需要一个指向浮动。 ％LF 需要一个指向双击，％LF 要求一个指向长双。
The %f format for scanf requires a pointer to float. %lf requires a pointer to double, %Lf requires a pointer to long double.
值复制到一个临时铸造（是这样吧？）
copying the value to a temp and casting(is this right?)
如果你提供了一个float参数，则实现创建一个临时的double类型的，用浮点值初始化它，并把它作为可变参数。顾名思义铸造的明确的通过使用强制转换运算符的转换 - 你可以，如果你为了使其准确清楚地向读者发生了什么事情，但像投浮动˚F = 3;的printf（％F，F）; 是完全一样的浮动F = 3;的printf（％F（双）F）; 。默认参数促销的含义投一样的。
If you provide a float argument, then the implementation creates a temporary of type double, initializes it with the float value, and passes this as the vararg. Casting by definition is explicit conversion by use of the cast operator -- you can cast if you like in order to make it exactly clear to the reader what's going on, but float f = 3; printf("%f", f); is exactly the same as float f = 3; printf("%f", (double)f);. The default argument promotion has the same meaning as the cast.
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