C++のビットフィールドと-Wconversion

Question

-Wconversionは、g ++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：私は（このファイル内を含む）私のプロジェクトの他の部分のために有効に-Wconversionたい

$ 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] 

。代入文をここで "修正"して警告を受け取らないようにするにはどうしたらいいですか？

Answer 1

変換がオーバーフローできないようにしてください。おそらくあなた暗黙的な変換は、あなたの価値を変更することができることがある

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); 
} 

Answer 2

Wconversionは毎回警告与える：ここでは一つの方法です。それは、あなたのコードには問題はないと言われています。

上記のようにstatic_cast<usigned_int>を使用すると、問題が解決されます。

Some more information