C++: error: narrowing conversion of 'x' from 'int' to 'char' inside { } [-Wnarrowing]

Meaning

List-initialisation was designed to catch lossy conversions. Constants that fit are allowed (char c{65};), but a variable or an out-of-range literal is an error in C++11 and later. Use the right type, or make the conversion explicit with static_cast when you really want it.

Common causes

• Initialising a smaller type from a variable with braces (int to char, long to int)
• Braced initialisation of an integer from a floating-point value
• Value outside the target type's range
• Signed to unsigned conversion of a negative or non-constant value in {}

Detailed fix by platform

Code examples

int n = 300;
// char c{n};                       // error: narrowing conversion of 'n' from 'int' to 'char'
char c{static_cast<char>(n)};       // OK, truncation is explicit (value 44)

double d = 3.9;
// int i{d};                        // error: narrowing conversion
int i = static_cast<int>(d);        // 3

How to diagnose

1. Types — Source and target types in the message?
2. Constant — Is the value a compile-time constant that fits?
3. Intent — Is truncation really wanted?

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