auto assignment in if clause

Question

I have code below and it reports "auto is not allowed here"

//GeSettings() returns boost::optional<ValueType> and could be empty

if((auto ret = GetSettings(InputField)) && ShouldWeDoThis())
{
   do something with ret;
}

but changing as below if fine.

if(auto ret = GetSettings(InputField))
{
    if(ShouldWeDoThis())
    {
        do something with ret;
    }
}

The reason behind could be silly buy may I ask why? I am using Visual Studio 2017

Answer 1

Your first version:

if((auto ret = GetSettings(InputField)) && ShouldWeDoThis())

is not allowed under any c++ standard, as you can not have declarations in expressions.

§6.4 (from c++11) Selection statements defines what an if statement looks like:

• if (condition) statement
• if (condition) statement else statement

Further down condition is defined:

• expression
• attribute-specifier-seq_opt decl-specifier-seq declarator = initializer-clause
• attribute-specifier-seq_opt decl-specifier-seq declarator braced-init-list

This allows us to write things like

if (auto ret = Foo())

However, in c++17 and forward, the definition changed (and moved to §9.4):

• if constexpr_opt (init-statement_opt condition) statement
• if constexpr_opt (init-statement_opt condition) statement else statement

We can now write things like:

if (auto ret=Foo(); ret && Bar())

Which should be useful for your purpose.