📄 习题5.18 .html
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<p class=aff2 style='margin-top:8.0pt;margin-right:0cm;margin-bottom:4.0pt;
margin-left:0cm;text-indent:6.0pt'>习题5.30 </p>
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<p class=XT>下列语句哪些（如果有的话）是非法的或错误的？</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(a)
vector<string> svec(10);</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(b)
vector<string> *pvec1 = new vector<string>(10);</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(c)
vector<string> **pvec2 = new vector<string>[10];</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(d)
vector<string> *pv1 = &svec;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(e)
vector<string> *pv2 = pvec1;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(f) delete svec;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(g) delete pvec1;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(h) delete [] pvec2;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(i) delete pv1;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(j) delete pv2;</p>
<p class=aff4>【解答】</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>错误的有(c)和(f)。</p>
<p class=MsoNormal style='text-indent:19.0pt;text-autospace:none'>(c)的错误在于：pvec2是指向元素类型为string的vector对象的指针的指针（即pvec2的类型为vector<string> **），而new操作返回的是一个指向元素类型为string的vector对象的指针，不能用于初始化pvec2。</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(f)的错误在于：svec是一个vector对象，不是指针，不能对它进行delete操作。</p>
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<p class=aff2 style='margin-top:8.0pt;margin-right:0cm;margin-bottom:4.0pt;
margin-left:0cm;text-indent:6.0pt'>习题5.31 </p>
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<p class=XT>根据5.12.2节的变量定义，解释在计算下列表达式的过程中发生了什么类型转换？</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(a) if (fval)</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(b) dval = fval +
ival;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:110%;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(c) dval + ival +
cval;</p>
<p class=XT>记住，你可能需要考虑操作符的结合性，以便在表达式含有多个操作符的情况下确定答案。</p>
<p class=aff4>【解答】</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(a) 将fval的值从float类型转换为bool类型。</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(b) 将ival的值从int类型转换为float类型，再将fval + ival的结果值转换为double类型，赋给dval。</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(c) 将ival的值从int类型转换为double类型，cval的值首先提升为int类型，然后从int型转换为double型，与dval + ival的结果值相加。</p>
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<p class=aff2 style='margin-top:8.0pt;margin-right:0cm;margin-bottom:4.0pt;
margin-left:0cm;text-indent:6.0pt'>习题5.32 </p>
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<p class=XT>给定下列定义：</p>
<p class=MsoNormal align=left style='text-align:left;line-height:9.0pt;
punctuation-trim:none;text-autospace:none'>char cval; int ival; unsigned int ui;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:9.0pt;
punctuation-trim:none;text-autospace:none'>float fval; double dval;</p>
<p class=XT style='text-indent:0cm'>指出可能发生的（如果有的话）隐式类型转换：</p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(a)
cval = 'a' + 3; (b) fval = ui – ival * 1.0;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:12.0pt;
layout-grid-mode:char;punctuation-trim:none;text-autospace:none'>(c) dval = ui * fval; (d) cval = ival + fval + dval;</p>
<p class=aff4>【解答】</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(a) 'a'首先提升为int类型，再将'a' + 3的结果值转换为char型，赋给cval。</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(b) ival转换为double型与1.0相乘，ui转换为double型再减去ival * 1.0的结果值，减操作的结果转换为float型，赋给fval。</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(c) ui转换为float型与fval相乘，结果转换为double型，赋给dval。</p>
<p class=MsoNormal align=left style='text-align:left;text-autospace:none'>(d) ival转换为float型与fval相加，结果转换为double型，再与dval相加，结果转换为char型，赋给cval。</p>
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<p class=aff2 style='margin-top:8.0pt;margin-right:0cm;margin-bottom:4.0pt;
margin-left:0cm;text-indent:6.0pt'>习题5.33 </p>
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<p class=XT>给定下列定义：</p>
<p class=MsoNormal align=left style='text-align:left;line-height:9.0pt;
punctuation-trim:none;text-autospace:none'>int ival; double dval;</p>
<p class=MsoNormal align=left style='text-align:left;line-height:9.0pt;
punctuation-trim:none;text-autospace:none'>const string *ps; char *pc; void *pv;</p>
<p class=XT style='text-indent:0cm'>用命名的强制类型转换符号重写下列语句：</p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(a)
pv = (void*)ps; (b) ival = int(*pc);</p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(c)
pv = &dval; (d) pc = (char*) pv;</p>
<p class=aff4>【解答】</p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(a) pv
= static_cast<void*> (const_cast<string*> (ps)); </p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(b) ival
= static_cast<int> (*pc);</p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(c) pv
= static_cast<void*> (&dval); </p>
<p class=MsoNormal align=left style='text-align:left;layout-grid-mode:char;
punctuation-trim:none;text-autospace:none'>(d) pc
= static_cast<char*> (pv);</p>
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