详谈signed 关键字
我们都知道且经常用到unsigned关键字，但有没有想过，与此对应的signed关键字有啥用？
inti=0; signedinti=0;
这俩有区别吗？没区别，看起来，signed完全是个累赘。
真的是这样吗？
我查阅了C++11的标准文档(草稿N3690)，发现一些端倪：
3.9.1Fundamentaltypes
Objectsdeclaredascharacters(char)shallbelargeenoughtostoreanymemberoftheimplementation'sbasiccharacterset.Ifacharacterfromthissetisstoredinacharacterobject,theintegralvalueofthatcharacterobjectisequaltothevalueofthesinglecharacterliteralformofthatcharacter.Itisimplementation-definedwhetheracharobjectcanholdnegativevalues.Characterscanbeexplicitlydeclaredunsignedorsigned.Plainchar,signedchar,andunsignedchararethreedistincttypes,collectivelycallednarrowcharactertypes.Achar,asignedchar,andanunsignedcharoccupythesameamountofstorageandhavethesamealignmentrequirements(3.11);thatis,theyhavethesameobjectrepresentation.Fornarrowcharactertypes,allbitsoftheobjectrepresentationparticipateinthevaluerepresentation.Forunsignednarrowcharactertypes,allpossiblebitpatternsofthevaluerepresentationrepresentnumbers.Theserequirementsdonotholdforothertypes.Inanyparticularimplementation,aplaincharobjectcantakeoneitherthesamevaluesasasignedcharoranunsignedchar;whichoneisimplementation-defined.
标准规定的很清楚，char,signedchar和unsignedchar是三种不同的类型。char会根据具体实现场景，而决定到底是signed还是unsigned.
再看看C11的标准文档(ISO/IEC9899:201x)呢？
6.7.2Typespecifiers
Eachofthecomma-separatedmultisetsdesignatesthesametype,exceptthatforbit-fields,itisimplementation-definedwhetherthespecifierintdesignatesthesametypeassignedintorthesametypeasunsignedint.
看来，bit-fields(位域)也存在同样的问题。(位域的概念可能也有点偏，经常写比较底层的接口或协议童鞋应该熟悉，可参考这里)
结论
在C/C++中，signed关键字绝大多数情况下都是累赘，但对于上述所言的两种情况，即在char与bit-fields的使用过程中，还是有比较隐晦的作用的。
给自己提个醒，总是好的。