我可以在一行上写一个带有变量声明的if语句吗？

Question

I was wondering if there was a way to put this on one line?

if (auto r = getGlobalObjectByName(word)) r->doSomething; // This works fine

if (!auto r = getGlobalObjectByName(word)) r->doSomething; // Says "expected an expression"

if (auto r = getGlobalObjectByName(word) == false) r->doSomething; // Also doesn't work.

I also tried surrounding it with extra brackets, but that doesn't seem to work. I find this really convenient doing it on one line.

Answer 1

Since C++17 you can use an initializer if-statement :

if (auto r = getGlobalObjectByName(word); !r) r->doSomething;

The semantics are:

if (init-statement; condition) statement

The only difference from the "traditional" if-statement is the init-statement , which initializes a variable in the block scope, similar to for-loops.

Answer 2

If you have C++17, use the if (init statement; condition) form. If not, you have three choices:

• Stop trying to keep this all on one line. For example:

   auto r = getGlobalObjectByName(word); if (!r) r->doSomething(); 

• Use an else :

   if (auto r = getGlobalObjectByName(word)) {} else r->doSomething(); 

(Note that this requires that r is a smart pointer with very strange semantics for the operator bool() function. OTOH, I assume this is actual a short piece of example code, rather than your actual code).

I think I would only use the else form if it was really important to keep everything on one line (to preserve a tabular formatting of the code for example).

Answer 3

What you are trying to do is fine. By defining the variable inside the if you restrict its scope . That helps to reduce the number of stray variables after they have served their purpose.

Using this technique, if you want to follow the negative path, you need to use else like this:

if(auto r = getGlobalObjectByName(word))
{
    r->doSomething();
}
else
{
    // r == nullptr
    // so do something else
}

Answer 4

There's also a way to do this with lambdas and C++14 but it does seem rather silly.

[](auto r){ if(!r)r->doSomething(); }(getGlobalObjectByName(word));

In C++11 you could also do this horrible mess (same idea, just no auto )

[](decltype(getGlobalObjectByName(word)) r){ if(!r)r->doSomething(); }(getGlobalObjectByName(word));

It is certainly no better than this clearer C++11 version mentioned by Martin Bonner :

{
    auto r = getGlobalObjectByName(word);
    if(!r)r->doSomething();
}

Here your code is clear that you want r to only be around for the duration of the if statement.

Answer 5

Before C++17 you can define a wrapper class, like here:

#include <utility>
template<typename T>
class NotT
{
    T t;
public:
    template<typename U>
    NotT(U&& u) : t(std::move(u)) {}
    explicit operator bool() const { return !t; }
    T      & value()       { return t; }
    T const& value() const { return t; }
};
template<typename T> NotT<T> Not(T&& t)
{
    return NotT<T>(std::move(t));
}

#include <memory>
#include <iostream>

int main()
{
    if(auto p=Not(std::make_shared<int>(2134)))
        std::cout << "!p: p=" << p.value().get() << '\n';
    else
        std::cout << "!!p: p=" << p.value().get() << ", *p=" << *p.value() << '\n';

}

Live example