关于第六章习题的一些疑问 about cppprimer HOT 8 CLOSED

template
T abs(T i)
{
return i >= 0 ? i : -i;
}

This is template, out of this chapter. (sorry for that.)

But the question of exercise 6.5:

Exercise 6.5: Write a function to return the absolute value of its argument.

Doesn't clarify which type of the argument. so in my opinion, template is the only way. maybe @Mooophy can give some advices.

The other question about Visual Studio 2013, I will respond to you tomorrow. (I don't have windows at home.) 😅

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我在想，既然您用模板，那么我直接用int替换成实体，这样算不算重复定义？vs2013编译的时候，第一次，提示错误，说重复定义，但是奇怪的是，第二次编译的时候竟然不报错了，不知道怎么回事，求解答！

I tried the codes below on vs2012 and qt creator with MinGW4.8.2. No gcc nor clang can be found at hand..

experiment I

#include <iostream>

template <typename T>
T abs(T i)
{
    std::cout << "template" << std::endl;
    return i >= 0 ? i : -i;
}

int abs(int i)
{
    std::cout << "non template" << std::endl;
    return i >= 0 ? i : -i;
}

int main()
{
    auto result = abs(-1);
    std::cout << result << std::endl;
    return 0;
}

Both compiled and output the same thing :

non template
1

So the non template version was called.

experiment II

#include <iostream>

template <typename T>
T abs(T i)
{
    std::cout << "template" << std::endl;
    return i >= 0 ? i : -i;
}

//int abs(int i)
//{
//    std::cout << "non template" << std::endl;
//    return i >= 0 ? i : -i;
//}

int main()
{
    auto result = abs(-1);
    std::cout << result << std::endl;
    return 0;
}

Output differed the previous try, but same on two compilers :

1

It's reasonable to guess that this time the library version abs was called.

experiment III

#include <iostream>

template <typename T>
T abs_def(T i)
//^^^^^^^<--note the function name had been modified.
{
    std::cout << "template" << std::endl;
    return i >= 0 ? i : -i;
}

//int abs(int i)
//{
//    std::cout << "non template" << std::endl;
//    return i >= 0 ? i : -i;
//}

int main()
{
    auto result = abs_def(-1);
    //            ^^^^^^^

    std::cout << result << std::endl;
    return 0;
}

output:

template
1

Finally the custom template version abs was called.

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A summary copied from here

• Nontemplate functions are first-class citizens. A plain old nontemplate function that matches the parameter types as well as any function template will be selected over an otherwise-just-as-good function template.
• If there are no first-class citizens to choose from that are at least as good, then function base templates as the second-class citizens get consulted next.

For more detail please go through cp5 chapter 16

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最后，int fact(int val)我在vs2013中是这么写的

#ifndef int fact(int val)
#endif

Pretty sure @Ocxs has used macro in a wrong way. Please read something like this.

Alternatively, you can try Qt Creator which generates header guards when new header file added. Visual Studio is a great tool, but not for beginners..

Hope helps.

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@Mooophy Awesome! 👍 I don't need to say anything more. 😺

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~

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@Mooophy Thank you for your advice .:+1:

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@pezy Thank you for your answer in SegmentFault!:+1:

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