Assign double to std::string — no compile error? [duplicate]

Answer 1

std::string has an overload for operator= that takes a character. When you pass an argument to a function by value (i.e, an operator), copy initialization occurs. In copy initialization, standard conversions, also known as an "implicit conversion", may be used to the convert the value. In this case, your double is being silently converted to a char so that it may be used in operator=.

For GCC, -Wall -Wextra -pedantic will not make a diagnostic appear. You can try -Wfloat-conversion, which is enabled by -Wconversion. Example:

main.cpp:11:10: warning: conversion to 'char' alters 'double' constant value 
[-Wfloat-conversion]
        a = 3.2;

Alternatively, use braces to force a narrowing conversion error.

s = {4.3};
// warning: narrowing conversion of '4.2e+1' from 'double' to 'char' inside { } 
// [-Wnarrowing]

Answer 2

The compiler will do an implicit conversion from 2 to the equivalent ASCII char: "start of text" (therefore you can't see anything).

Maybe you have disabled the warning of your compiler. Try to turn them on.

Answer 3

GCC:

compile in gcc with option [-Wconversion], then you'll see that the compiler will issue the warning:

warning: conversion to 'char' alters 'double' constant value [-Wfloat-conversion]

LIVE DEMO

MSVC don't seem to be concerned about this at all:

VS2013 produces the following warning:

warning C4244: 'argument' : conversion from 'double' to 'char', possible loss of data

even though it is clearly wrong and doesn't actually work anyway!

It's not wrong, it's just an implicit conversion from the truncated double 2.3 to a char with ASCII code 2 (start of text).