Templated class with constructor arguments

Question

Consider this class:

template <typename t>
class Something
{
public:
    Something(t theValue) {mValue=theValue;}
    t mValue;
};

When I do this, everything is okay:

Something<float>* mSomething=new Something<float>(100);

HOWEVER, if I try to do this:

Something<float> mSomething(100);

I am declaring the above inside another class-- i.e.

class SomethingElse
{
public:
     Something<float> mSomething(100);
};

It tells me that anything I put inside the parenthesis is a syntax error.

What exactly is the necessary syntax here-- or is this some quirk of templates and thus not possible?

Fail code example here: https://wandbox.org/permlink/anaQz9uoWwV9HCW2

Answer 1

You may not use such an initializer as a function call

class SomethingElse
{
public:
     Something<float> mSomething(100);
};

You need to use an equal or braced initializer. For example

class SomethingElse
{
public:
     Something<float> mSomething { 100 };
};

Here is a demonstration program.

template <typename t>
class Something
{
public:
    Something(t theValue) {mValue=theValue;}
    t mValue;
};

class SomethingElse
{
public:
     Something<float> mSomething{ 100 };
};

int main()
{
}

According to the C++ grammar

member-declarator:
    declarator virt-specifier-seqopt pure-specifieropt
    declarator brace-or-equal-initializeropt
    identifieropt attribute-specifier-seqopt: constant-expression

See the term brace-or-equal-initializer.