Why do anonymous objects sometimes require a default constructor?

Question

If I write the following program, it works as I expect:

struct Foo {
    Foo (std::string x) { std::cout << x << std::endl; }
};

int main () { Foo("hello, world"); }

However, if I write a slightly different program, I get a compilation error:

struct Foo {
    Foo (std::string x) { std::cout << x << std::endl; }
};

std::string x("hello, world");

int main () { Foo(x); }

The error is:

prog.cc: In function 'int main()':
prog.cc:10:20: error: no matching function for call to 'Foo::Foo()'

The complete error can be seen on IDEONE.

Why does the error occur for the second program and not the first?

Answer 1

You have declared a variable x with type Foo

struct Foo {
    Foo(){}
    Foo (std::string x) { std::cout << x << std::endl; }
    void test(){ std::cout << "test" << std::endl; };
};

std::string x("hello, world");

int main () { Foo(x); x.test(); }

print "test"

---

What you want is use uniform initialization syntax Foo{x}

struct Foo {
    Foo (std::string x) { std::cout << x << std::endl; }
};

std::string x("hello, world");

int main () { Foo{x}; }

print "hello, world"