习题答案 及 StackOverflow上的讨论

更新：

不少同学要代码。我折腾了一晚上，终于把习题的代码发到github上了：
https://github.com/Mooophy/Cpp-Primer

要讨论的同学要熟悉一下git的用法，需要以下几步：
1、注册帐号；
2、fork 我的repository；
3、在你自己的repository里commit，比如填加个注释什么的；
4、给我一个pull request。

==========================================
或者到这个小组：http://www.douban.com/group/532124/
==========================================

git的操作大概需要一两天来上手，但换来的价值远远超过读一天CP或者做一天习题。因为github上实在是神器遍地、大牛横行啊。。

详细内容参阅这里：
Fork A Repo
https://help.github.com/articles/fork-a-repo

Using pull requests
https://help.github.com/articles/using-pull-requests





王月
08， 2014

===========以下为原帖===========

3-16章的习题基本都有做，有需要的同学可以豆邮我。
以下是部分po在Stack Overflow的习题，都有不错的讨论，值得一看，更欢迎指正。


Exercise 4.33: Explain what the following expression does(Page158):
someValue ? ++x, ++y : --x, --y

http://stackoverflow.com/questions/22828628/how-does-the-compiler-interpret-this-expression-in-terms-of-precedence-and-asso

Exercise 6.33: Write a recursive function to print the contents of a vector.
http://stackoverflow.com/questions/20184299/how-to-understand-and-fix-the-segmentation-fault-in-this-code


Exercise 10.24:Use bind and check_size to find the first element in a vector of ints that has a value greater than the length of a specified string value.
http://stackoverflow.com/questions/20539406/what-type-does-stdfind-if-not-return



Exercise 11.11: Redefine bookstore without using decltype.
http://stackoverflow.com/questions/20627530/how-to-define-a-multiset-using-a-function-pointer



Exercise 11.16: Using a map iterator write an expression that assigns a value to an element.(Page 431)
http://stackoverflow.com/questions/20675892/is-it-possible-to-assign-a-value-to-an-element-of-map-using-a-map-iterator


Exercise 12.3: Does this class need const versions of push_back and pop_back? If so, add them. If not, why aren’t they needed? (Page 458)
http://stackoverflow.com/questions/20725190/operating-on-dynamic-memory-is-it-meaningful-to-overload-a-const-memeber-functi


Exercise 13.22: Assume that we want HasPtr to behave like a value. That is, each object should have its own copy of the string to which the objects point. We’ll show the definitions of the copy-control members in the next section. However, you already know everything you need to know to implement these members. Write the HasPtr copy constructor and copy-assignment operator before reading on.(Page 511)
http://stackoverflow.com/questions/20864892/is-it-a-good-practice-to-point-to-a-new-address-on-free-store-when-dynamicly-all


Exercise 13.50: Put print statements in the move operations in your String class and rerun the program from exercise 13.48 in § 13.6.1 (p. 534) that used a vector to see when the copies are avoided.(P.544)
http://stackoverflow.com/questions/20967732/why-wasnt-the-move-constructor-called


Exercise 13.53: As a matter of low-level efficiency, the HasPtr assignment operator is not ideal. Explain why. Implement a copy-assignment and move-assignment operator for HasPtr and compare the operations executed in your new move-assignment operator versus the copy-and-swap version.(P.544)
http://stackoverflow.com/questions/21010371/why-is-it-not-efficient-to-use-a-single-assignment-operator-handling-both-copy-a


Exercise 14.7: Define an output operator for you String class you wrote for the exercises in § 13.5 (p. 531).(Page 558)
http://stackoverflow.com/questions/21041877/why-doesnt-stdfor-each-lambda-work-as-expected


Exercise 14.14: Why do you think it is more efficient to define operator+ to call operator+= rather than the other way around?(P.561)
http://stackoverflow.com/questions/21071167/why-is-it-more-efficient-to-define-operator-to-call-operator-rather-than-the


Why isn't it a reference type when std::initializer_list being used as an assignment parameter?
http://stackoverflow.com/questions/21104923/why-isnt-it-a-reference-type-when-stdinitializer-list-being-used-as-an-assign


Exercise 14.26: Define subscript operators for your StrVec, String, StrBlob, and StrBlobPtr classes.
http://stackoverflow.com/questions/21197378/why-did-the-compiler-output-differently-when-compiling-two-similar-classes


Exercise 14.33: How many operands may an overloaded function-call operator take?
http://stackoverflow.com/questions/21211889/how-many-operands-may-an-overloaded-function-call-operator-take



Chapter 15:

The difference between reference and pointer as to being used for dynamic binding.
http://stackoverflow.com/questions/21278170/why-didnt-the-dynamic-binding-behave-as-expected

Trying to understand dynamic binding and virtual functions
http://stackoverflow.com/questions/21320779/trying-to-understand-dynamic-binding-and-virtual-functions

Chapter 16
Exercise 16.26: Assuming NoDefault is a class that does not have a default constructor, can we explicitly instantiate vector? If not, why not?
http://stackoverflow.com/questions/21525169/while-explicitly-instantiating-vectorsometype-what-is-the-sometype-default-co
答主全站排名73。由于排在前面的多是Java，C#或者C的，单算C++的话应该是top10左右的高手了，回答的很详细。


How to use functor as a member in class template?
http://stackoverflow.com/questions/21548259/how-to-use-functor-as-a-member-in-class-template


Exercise 16.27: For each labeled statement explain what, if any, instantiations happen. If a template is instantiated, explain why; if not, explain why not. P.677
http://stackoverflow.com/questions/21598635/how-is-a-template-instantiated
哈哈，这道题拿到了9个up vote，里面有讨论assertion的用法。我接受的那个回答讲的蛮好，深入浅出，值得一看，目前这个答案拿到了6个up。


Exercise 16.45: Given the following template, explain what happens if we call g on a literal value such as 42. What if we call g on a variable of type int? P.690
http://stackoverflow.com/questions/21624016/when-a-lvalue-is-passed-to-t-what-will-happen


Exercise 16.62: Define your own version of hash<Sales_data> and define an unordered_multiset of Sales_data objects. Put several transactions into the container and print its contents.

这道题没贴SO，但挺值得一说的。
官网给出了这道题的代码，但那个代码无法编译，而且把template 的specialization放在了.cc文件而非.h文件里。这样就违反了【template的所有代码都应放在header里】这一大原则。
另外，无论是课文还是官网的代码都没有inline下面这个operator的implementation，这也导致编译器报错：【重复定义】。

size_t hash<Sales_data>::operator()(const Sales_data& s) const

我最后是把这段implementation放到了class内，再把整段template的specialization放到Sale_data.h里的最后面才通过编译。经测试，代码可以正常运行。
请大牛指正。




Chapter 17

正则表达式：
http://stackoverflow.com/questions/22263224/how-to-use-stdregex