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This article introduces the knowledge of "how to avoid harmful arithmetic conversion on C++". In the operation of actual cases, many people will encounter such a dilemma. Then let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

ES.46: avoid lossy (narrowing, truncating) arithmetic conversion

Reason (reason)

A narrowing conversion destroys information, often unexpectedly so.

Narrowing conversion destroys information, usually not an expected action.

Example, bad (negative example)

A key example is basic narrowing:

The main examples illustrate the basics of narrowing:

Double d = 7.9

Int I = d; / / bad: narrowing: I becomes 7

I = (int) d; / / bad: we're going to claim this is still not explicit enough

Void f (int x, long y, double d)

{

Char C1 = x; / / bad: narrowing

Char c2 = y; / / bad: narrowing

Char c3 = d; / / bad: narrowing

}

Note (Note)

The guidelines support library provides a narrow_cast operation that can be used to indicate that narrowing is acceptable; a narrow ("if narrowing conversion occurs") operation that throws an exception when any information is lost.

I = narrow_cast (d); / / OK (you asked for it): narrowing: I becomes 7

I = narrow (d); / / OK: throws narrowing_error

We also include lossy arithmetic casts, such as from a negative floating point type to an unsigned integral type:

These two operations can also handle lossy arithmetic conversions, such as conversions from negative floating point numbers to unsigned integers.

Double d =-7.9

Unsigned u = 0

U = d; / / BAD

U = narrow_cast (d); / / OK (you asked for it): U becomes 4294967289

U = narrow (d); / / OK: throws narrowing_errorEnforcement (implementation recommendation)

A well-implemented code analyzer can detect all narrowing transformations. However, identifying all narrowing transformations can lead to a large number of false positive results. Recommendations:

Flag all floating-point to integer conversions (maybe only float- > char and double- > int. Here be dragons! We need data).

Mark all floating point to integer conversions (or just float to char and double to int. It's possible! We need data)

Flag all long- > char (I suspect int- > char is very common. Here be dragons! We need data).

Mark all long-to-char conversions (I suspect int-to-char conversions are common. It's possible! We need data)

Consider narrowing conversions for function arguments especially suspect.

Narrowing conversions of function parameters are particularly suspicious.

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