I learnt about curly-brace-delimited initializer in The C++ Programming Language, 4th ed. > Chapter 2: A Tour of C++: The Basics.
I am quoting from the book below.
The = form is traditional and dates back to C, but if in doubt, use the general {} -list form (§6.3.5.2). If nothing else, it saves you from conversions that lose information (narrowing conversions; §10.5):
int i1 = 7.2; // i1 becomes 7 int i2 {7.2}; // error : floating-point to integer conversion int i3 = {7.2}; // error : floating-point to integer conversion (the = is redundant)
However, I am unable to reproduce these results.
I have the following code.
#include <iostream>
int main()
{
int i1 = 7.2;
int i2 {7.2};
int i3 = {7.2};
std::cout << i1 << "\n";
std::cout << i2 << "\n";
std::cout << i3 << "\n";
}
When I compile and run it, I don't get any error. I get a warning about std=c++11 but no error.
$ g++ init.cpp
init.cpp: In function ‘int main()’:
init.cpp:6:12: warning: extended initializer lists only available with -std=c++11 or -std=gnu++11
int i2 {7.2};
^
$ ./a.out
7
7
7
Further, the warning is only for the second assignment but there is no warning for the third assignment. This seems to indicate that the = is not really redundant as mentioned in the book. If = were redundant, either both the second and third assignments would have produced warnings or both would not have produced warnings.
Then I compile them with the -std=c++11 flag.
$ g++ -std=c++11 init.cpp
init.cpp: In function ‘int main()’:
init.cpp:6:16: warning: narrowing conversion of ‘7.2000000000000002e+0’ from ‘double’ to ‘int’ inside { } [-Wnarrowing]
int i2 {7.2};
^
init.cpp:7:18: warning: narrowing conversion of ‘7.2000000000000002e+0’ from ‘double’ to ‘int’ inside { } [-Wnarrowing]
int i3 = {7.2};
^
$ ./a.out
7
7
7
Still no error. Only warnings. Although in this case the second and third assignments behave identically with respect to generating warnings.
So my question is: Although the book mentions that the second and third assignments are errors, why doesn't this code fail to compile?
i1andi3because this rule about narrowing conversions was introduced in C++11, and you had the compiler set to pre-C++11 mode. – Deoxyribonuclease