10

-Wconversiong++ でビット フィールドに値を代入すると、警告が表示されます。

ソースファイル：

struct Foo
{
public:
    unsigned int x : 4;
    unsigned int y : 9;
    unsigned int z : 17;
};

int main(int, char**)
{
    int a = 12;
    Foo f;
    f.x = a;
    f.x = (unsigned int)a;
    f.x = (unsigned char)a;
    f.x = (unsigned short)a;
    f.x = (unsigned)a;

    f.y = a;
    f.y = (unsigned int)a;
    f.y = (unsigned char)a; // no warning, sizeof(char) < 9
    f.y = (unsigned short)a;
    f.y = (unsigned)a;

    f.z = a;
    f.z = (unsigned int)a;
    f.z = (unsigned char)a; // no warning, sizeof(char) < 17
    f.z = (unsigned short)a; // no warning, sizeof(char) < 17
    f.z = (unsigned)a;
}

コンパイル出力:

$ g++ --version
g++ (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
<snip>
$ g++ -Wconversion test.cpp
test.cpp: In function ‘int main(int, char**)’:
test.cpp:13:8: warning: conversion to ‘unsigned char:4’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:14:22: warning: conversion to ‘unsigned char:4’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:15:23: warning: conversion to ‘unsigned char:4’ from ‘unsigned char’ may alter its value [-Wconversion]
test.cpp:16:24: warning: conversion to ‘unsigned char:4’ from ‘short unsigned int’ may alter its value [-Wconversion]
test.cpp:17:18: warning: conversion to ‘unsigned char:4’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:19:8: warning: conversion to ‘short unsigned int:9’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:20:22: warning: conversion to ‘short unsigned int:9’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:22:24: warning: conversion to ‘short unsigned int:9’ from ‘short unsigned int’ may alter its value [-Wconversion]
test.cpp:23:18: warning: conversion to ‘short unsigned int:9’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:25:8: warning: conversion to ‘unsigned int:17’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:26:22: warning: conversion to ‘unsigned int:17’ from ‘unsigned int’ may alter its value [-Wconversion]
test.cpp:29:18: warning: conversion to ‘unsigned int:17’ from ‘unsigned int’ may alter its value [-Wconversion]

-Wconversionプロジェクトの他の部分 (このファイル内を含む) で有効にしたい。ここで割り当てステートメントを「修正」して、警告が表示されないようにするにはどうすればよいですか?

4

2 に答える 2

15

変換がオーバーフローしないことを確認してください。1 つの方法を次に示します。

struct Foo
{
public:
    unsigned int x : 4; 
    unsigned int y : 9; 
    unsigned int z : 17;
};

int main(int, char**)
{
    int a = 12;
    Foo f;
    f.x = static_cast<unsigned int>(a & 15);    
    f.y = static_cast<unsigned int>(a & 511);   
    f.z = static_cast<unsigned int>(a & 131071);
}
于 2012-04-17T13:16:25.273 に答える
-1

Wconversionは、暗黙の変換によって値が変更される可能性があるたびに警告を発します。そうは言っても、コードに問題はありません。

上記static_cast<usigned_int>のように使用すると、問題が解決します。

いくつかの詳細情報

于 2012-04-17T13:23:46.070 に答える