c ++位字段和-Wconversion

时间:2012-04-17 13:07:54

标签: c++ g++ warnings bit-fields

当我使用g ++为位字段赋值时,

-Wconversion正在产生警告。

源文件:

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;
}

编译输出:

$ 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]

我想为项目的其他部分启用-Wconversion(包括在此文件中)。如何在此处“修复”赋值语句,以便我不会收到警告?

2 个答案:

答案 0 :(得分:14)

确保转换不会溢出。这是一种方式:

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);
}

答案 1 :(得分:-1)

每次有可能隐式转换可能会改变您的价值时,Wconversion会发出警告。话虽如此,你的代码没有问题。

如上所述,使用static_cast<usigned_int>可以解决您的问题。

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