Type Conversions

Implicit Conversions

• Type conversions happen automatically when we use an object of one type where an object of another type is expected.
• if can, the small type operands are generally promoted to a lager type. (preseve precision)

Integral Promotions

The integral promotions convert the small integral types to a larger integral type.

char c=1;
cout << sizeof(c+c) << endl;    // 4

在这样一个expression中，就算两个operands都是char，也会promote为int

Operands of Unsigned Type

• 如果signedness相同，那就老派方法promote
• 不同的话，只有当unsigned能够fit in signed operand时，unsigned变signed，不然都是signed变unsigned

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Conversions of 🔗Signed and Unsigned Types

unsigned：

• If we assign an out-of-range value to an object of unsigned type, the result is the remainder of the value modulo the number of values the target type can hold
• If we assign an out-of-range value to an object of signed type, the result is undefined.

所以，signed 可以转 unsigned，反之不行
当一个 expression 包含这俩类型时，the int value is converted to unsigned before the addition is done.

当赋值时花括号不允许narrowing conversion

Other Implicit Conversions

Array to Pointer

This conversion is not performed when an array is used with decltype, &, sizeof, typeid and initialize a reference to an array.

a similar pointer conversion happens when we use a function type in an expression.

Pointer Conversions

• 0, nullptr converted to any pointer type
• pointer convert to its void*

Conversions to bool

没啥好说的，就是在条件判断中，arithmetic和pointer在0啊null啊的时候会变为false

Conversion to const

就是在pointer和reference中提到的low-level const，可以对一个nonconst的做一个pointer to const。这中间暗含了把原来的nonconst转为const

Conversions Defined by Class Types

就比如string可以完全适应C-style character string，istream会变成bool

Explicit Conversions

虽然有时候是必要的，但是casts还是内在危险的

Named Casts

static_cast

就是一般的转换，包括int2float，void 和指针间的转换

当是指针之间的转换时，这个cast确保了指针的值是被保留的，但是如果没有匹配的时候用是undefined：

double d = 0.28976;  
void *p = &d;  
cout << *(static_cast<double*>(p));    // 0.28976

Use for Explicit Class-Type Conversions

item.combine(static_cast<Sales_data>(cin));

const_cast

Changes only a ==low-level const== in its operand. (也就是说，只能用在pointer和reference上) “casts away the const”

⚠️ if the original object is nonconst, then we casts away the const and obtain write access is legal. However, if the underlain is originally a const, the access is undefined.

与 static_cast 职能不互通

🔗const_cast and Overloading

reinterpret_cast

A reinterpret_cast generally performs a low-level reinterpretation of the bit pattern of its operands

是高阶操作，要深刻理解才用，之后遇到了再说吧