C++ Bitfelder und -Wwandlung

Question

-Wconversion erzeugt Warnungen, wenn ich einem Bitfeld mit g ++ einen Wert zuweise.

Quelldatei:

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; 
} 

Compilation Ausgabe:

$ 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] 

I -Wconversion für andere Teile meines Projekts (einschließlich in dieser Datei) aktiviert werden soll. Wie repariere ich die Zuweisungsanweisungen hier, damit ich keine Warnungen bekomme?

Answer 1

Sicherstellen, dass die Konvertierung nicht überlaufen kann. Hier ist eine Möglichkeit:

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 gibt jedes Mal Warnung gibt es eine möglicherweise, dass Ihre implizite Konvertierung Ihren Wert verändern könnte. Das wurde gesagt, es gibt kein Problem mit Ihrem Code.

Die Verwendung von static_cast<usigned_int>, wie oben erwähnt, wird Ihr Problem beheben.

Some more information